Leech Life does slightly more damage, but Drain Punch has more coverage.
Drain Punch.
Although typically you shouldn't use either, if you have to choose one of the two, better coverage is the way to go. At +2, the little difference in base damage is negligible, as you'll OHKO or 2HKO a good number of opponents. Also, obviously, a super effective Drain Punch will more than make up for the small drop in power than a neutral Leech Life.