16, 32, 64 GB iPhone / iPod touch: are there any negative effects of size?
It shouldn't have any impact on performance or battery life.

I believe the same number of flash memory chips are used in the 32 GB and 64 GB models, just in different capacities, so the power drain would be almost identical. Either way, flash memory  takes up very little power compared to things like the screen and wireless radios, so any impact would almost certainly be unnoticeable.

As for performance, I haven't seen any metrics on it, but iOS performance isn't really affected by device capacity. The possible exception here is when you have very little free space, but that's just a guess.

Bottom line, I would suggest not worrying about it - just buy the capacity based on your budget and needs.