Odd you ask that, since both are HORRIBLE, ... especially often draining your battery low.
General consideration of your MacBook battery
Contrary to popular myths about notebook batteries, there is protection circuitry in your Macbook and therefore you cannot ‘overcharge’ your notebook when plugged in and already fully charged.
However if you do not plan on using your notebook for several hours, turn it off (plugged in or otherwise), since you do not want your Macbook ‘both always plugged in and in sleep mode’.
A lot of battery experts call the use of Lithium-Ion cells the "80% Rule", meaning use 80% of the full charge or so, then recharge them for longer overall life. The only quantified damage done in the use of Lithium Ion batteries are instances where the internal notebook battery is “often drained very low”, this is bad general use of your notebook battery.
A person who has, for example, 300 charge cycles on their battery and is recharging at say 40% remaining of a 100% charge has a better battery condition state than, say, another person who has 300 charge cycles on their battery and is recharging at say 10-15% remaining on a 100% charge. DoD (depth of discharge) is much more important on the wear and tear on your Macbook’s battery than the count of charge cycles. There is no set “mile” or wear from a charge cycle in specific. Frequent high depth of discharge rates (draining the battery very low) on a Lithium battery will hasten the lowering of maximum battery capacity.
All batteries in any device are a consumable meant to be replaced eventually after much time, even under perfect use conditions.
If the massive amount of data that exists on lithium batteries were to be condensed into a simplex, helpful, and memorable bit of information it would be:
1. While realistically a bit impractical during normal everyday use, a lithium battery's longevity and its chemistry's health is most happy swinging back and forth between 20% and 85% charge roughly.
2. Do not purposefully drain your battery very low (10% and less), and do not keep them charged often or always high (100%).
3. Lithium batteries do not like the following:
A: Deep discharges, as meaning roughly 10% or less on a frequent basis.
B: Rapid discharges as referring to energy intensive gaming on battery on a frequent basis (in which case while gaming, if possible, do same on power rather than battery). This is a minor consideration.
C: Constant inflation, as meaning always or most often on charge, and certainly not both in sleep mode and on charge always or often.
From Apple on batteries:
"Apple does not recommend leaving your portable plugged in all the time."
Keep it plugged in when near a socket so you keep the charging cycles down on your LiPo (lithium polymer) cells / battery, but not plugged in all the time. When not being used for several hours, turn it off.
DoD (depth of discharge) is far more important on the wear and tear on your Macbook battery than any mere charge cycle count. *There is no set “mile” or wear from a charge cycle in general OR in specific. As such, contrary to popular conception, counting cycles is not conclusive whatsoever, rather the amount of deep DoD on an averaged scale of its use and charging conditions.
(as a very rough analogy would be 20,000 hard miles put on a car vs. 80,000 good miles being something similar)
*Contrary to some myths out there, there is protection circuitry in your Macbook and therefore you cannot overcharge it when plugged in and already fully charged
*However if you don’t plan on using it for a few hours, turn it OFF (plugged in or otherwise) ..*You don’t want your Macbook both always plugged in AND in sleep mode (When portable devices are charging and in the on or sleep position, the current that is drawn through the device is called the parasitic load and will alter the dynamics of charge cycle. Battery manufacturers advise against parasitic loading because it induces mini-cycles.)
Keeping batteries connected to a charger ensures that periodic "top-ups" do very minor but continuous damage to individual cells, hence Apples recommendation above: “Apple does not recommend leaving your portable plugged in all the time”, …this is because “Li-ion degrades fastest at high state-of-charge”.
This is also the same reason new Apple notebooks are packaged with 50% charges and not 100%.
Contrary to what some might say, Lithium batteries have an "ideal" break in period. First ten cycles or so, don't discharge down past 40% of the battery's capacity. Same way you don’t take a new car out and speed and rev the engine hard first 100 or so miles.
Proper treatment is still important. Just because LiPo batteries don’t need conditioning in general, does NOT mean they dont have an ideal use / recharge environment. Anything can be abused even if it doesn’t need conditioning.
Storing your MacBook
If you are going to store your MacBook away for an extended period of time, keep it in a cool location (room temperature roughly 22° C or about 72° F). Make certain you have at least a 50% charge on the internal battery of your Macbook if you plan on storing it away for a few months; recharge your battery to 50% or so every six months roughly if being stored away. If you live in a humid environment, keep your Macbook stored in its zippered case to prevent infiltration of humidity on the internals of your Macbook which could lead to corrosion.
Your battery is subject to chemical aging even if not in use. A Lithium battery is aging as soon as its made, regardless.
In a perfect (although impractical) situation, your lithium battery is best idealized swinging back and forth between 20 and 85% SOC (state of charge) roughly.
Further still how you discharge the battery is far more important than how it is either charged or stored short term, and more important long term that cycle counts.
Ultimately counting charge cycles is of little importance. Abuse in discharging (foremost), charging, and storing the battery and how it affects battery chemistry is important and not the ‘odometer’ reading, or cycle counts on the battery.
Everything boils down to battery chemistry long term, and not an arbitrary number, or cycle count.
Keep your macbook plugged in when near a socket since in the near end of long-term life, this is beneficial to the battery.
In a lithium battery, deep discharges alter the chemistry of the anode to take up lithium ions and slowly damages the batteries capacity for the cathode to transport lithium ions to the anode when charging, thereby reducing max charge levels in mAh. In short, radical swings of power to lithium cells disrupts the chemical ecosystem of the battery to hold charges correctly which likewise impedes the perfect transfer of lithium ions both in charging and discharging. In charging your lithium battery, lithium ions are “pushed uphill” (hard) to the anode, and discharged “downhill” (easy) to the cathode when on battery power. Deep discharges, damages this “upward” electrolyte chemistry for the battery to maintain a healthy charge and discharge balance relative to its age and cycles.
Optimally, in terms of a healthy lithium battery and its condition, it is most happy at 50% between extremes, which is why low-power-drain processors such as the Haswell are ideal on lithium battery health since a partially charged battery with a low-drain processor has, in general, much more usage in hours
Battery calibration, battery memory, battery overcharging, battery training, …all these concepts are mostly holdovers from much older battery technology, and on older Apple portable Macbooks ranging from early nicads, NiMh and otherwise; and these practices do not apply to your lithium battery and its smart controllers.
Calibrating the battery on older Apple portable Macbooks with removable batteries.
There is no calibration of current Apple portable Macbooks with built-in batteries.
There is no battery calibration with current Apple portable Macbooks with built-in batteries. Lithium batteries have essentially a 0-‘memory’, and all such calibration involve the estimations fed to the system controller on the SOC (state of charge) of the battery over long periods of time as the battery degrades. The software based battery controller knows the battery's characteristics, or SOC and adjusts itself. This is why there is both no need and purpose to periodically deeply drain your macbook battery, since it doesn’t affect the characteristics of the battery, and further still deep discharges are something you should not do on purpose to any lithium battery.
From BASF: How Lithium Batteries work