I don't know which MPB you have, but I know of one person who measured his MBP and stated that it is 22ohms; if that's the case, the same headphones should be good for both of your Macs.
Another way to approach it is to use something like this:
I have a 32Ω headphone and the audio volume is perfect. I tried a 70Ω headphone and to it's impossible to listen a song the volume is tooooo quite.
It's difficult to find some professional headphone with 32Ω.
I found the Sony MDR-V700DJ with 24Ω and the Shure SRH840 with 44Ω.
Wich one would you choose? 24Ω it's loud 44Ω it's loud. It's an hard choise.
I have to use it also with the iphone
FYI, I got a macbook pro late 2008.
I got several headphones :
- sennheiser HD 25 SP, impedance 85 ohms, I got no pb with sound, loud and clear. (and absolutly no problem with my iPod)
- sony md4509HD, impedance 24 ohms, sound ok but I can hear "ground noise" (must have a poor electrical installation or maybe it is because of my PLCs). anyway, it makes it useless !!!
So, be careful with low impedance and ground noise.