>>For punching paper, what is better? 38s or 357s?
Depends on your paper-punching objectives. To develop extreme accuracy and cure your flinch, a lot of mild .38s is best (add a lot of ball-and-dummy, too). But if your objective is to train for real self-defense, you should run something like the IDPA classifier once or twice a year with your "real" ammo, with some extreme close-in and low-light exercises probably four times a year. Minimum.
>>Is there a difference in the wear and tear on a gun between 38s & 357s?
Probably not noticeable in the first 10,000 rounds. Will you shoot it enough to make it a problem? If you're spending that much on ammo over only a few years, don't worry about it and just budget yourself for a replacement gun every so often. At the really cheap rate of $10/100 rounds, 10K is a thousand bucks in ammo.
Better to wear out than to rust out. As the industrial espionage geek said to Dobson in _Jurassic Park_, "Don't get cheap on me."
>>Is there an accuracy differences between 38s & 357s?
Depend on the witchcraft of guns and ammo. I've seen it go both ways--some do better with .38s, some with .357s, and EVERY revolver shoots better with some .357s than some .38s, and vice versa.