Q: If I'm not going to use my watch for 1-2 days, is Power Reserve good to put it in rather than keeping the watch on the charger?

Hi

That is really a matter of personal preference.

Power Reserve mode is intended for use when you are still wearing the watch. It disables all other features except telling the time, in order to keep this function available for as long as possible when the battery charge has fallen to a low level. As you are not using the watch during these periods, there is no benefit to be gained from using this mode instead of turning the watch off (the same action is also required in each case to restart the watch afterwards).

Whilst Apple Watch cannot be overcharged (charging starts and stops automatically), a full charge only requires up to 2.5 hours.

Of your three options, you may prefer to just turn the watch off after charging it:

Restart your Apple Watch - Apple Support