Evolve or power up?

Power up for sure. Especially if that's 15 attack. Congrats!

Psychic always nice for those players who try to speed K.O. your Blissey with Machamp.

Power up the Blissey you have. Despite the similar IV%, the one already have evolved would be 41CP higher than the other one at level 30, and that gap will only grow if you ever power up past that.

Blisseys often need to be powered up to get a decent gym position, so how high CP it will reach is generally far more important than IVs, and the two can be quite different. For example, a 58% IV 15/11/0 Blissey will reach a higher CP than your 89% IV 12/14/14. That latter one doesn't look worthwhile, given it's mediocre CP potential..If it were me, I wouldn't even consider powering it up.

What I look for in a Chansey is 15 attack + very high defense. I found one of my best ones ever yesterday, which was a 80% 15 attack + 15 defense + 6 stamina. It will reach a approximately the same CP as a 98% IV 14/15/15.

Invest in the one with better attack.