I've been meaning to play the whole quartet of games for some time now and just looked into buying it. These appear to be my choices, given that I use a Mac 99% of the time, but strongly favour Steam over the Mac App Store, and don't own any consoles:
Episode 1: Steam (Windows), Mac App Store
Episode 2: Steam (Windows), Mac App Store
Episode 3: Steam (Mac and Windows), Mac App Store, iOS
Episode 4: Steam (Windows)
My 2 choices appear to be:
Buy it all on Steam (pro: achievements, con: rebooting)
Buy Episodes 1 & 2 on Mac App Store (pro: cheaper even without sale, and there is a sale on Ep 2 right now), and Episodes 3 & 4 on Steam with the hope that 4 will eventually be cross-platform.
One easy deciding factor would be the save game carrying-over thing which I've found inconclusive evidence of. Does it work between all episodes, or only 1-to-2 and/or 3-to-4? Does it work across platforms (i.e. is the save game a standard file I can move from a Steam install to the Mac App Store equivalent)? I gather that when it does work, the impact on the receiving Episode may be minimal, but I savour the complete experience when it comes to gaming product.