Blissey with Psychic

ZH/P > P/P

Though neither are great, I'd power up the P/P Blissey. Your ideal attacker, Machamp, is always dodging the Zen Headbutts and Pound has similar timing so it's easy to dodge with that combo. However, against a P/P Blissey, players using Machamp usually elect not to dodge Pound attacks. That makes dodging Psychic extremely difficult, resulting in a KO more often than not.

Power up the one with greater attack ivs.
I have a 15/15/6 (a/d/s) and it sits higher than 13/15/15 blisseys of equivalent level.