Steam all day, especially after this disaster of expired certificates this year [0]. I had a copy of a game from 2007 that will probably never be touched by the developers again, which leaves me $20 in the hole with a broken game. I bought it again from Steam, works better than the App Store version ever did.
[0]: http://www.macrumors.com/2015/11/17/apple-responds-mac-app-s...