The room 3, definitely. Prism was pretty but that was about it. The room 3 was pretty, longer, and had more variety. Plus, the puzzles in the room 3 weren't just abstractly linked to the game. I don't think these games are worth comparing, though. The production team behind each one is wildly, wildly, different, and the intended effect of each game is also very different. They're both worth getting in their own right. And to be absolutely honest I wasn't 100% in love with either - but I'd still recommend em both.
Understand. I am currently immersed in a number of apps, but was thinking of some new puzzle diversion. I thoroughly enjoyed Room 1 and 2, but for various reasons have not got #3 yet.