I heard portal 2 is great, but you have to have played and completed portal 1, I also heard crysis is very CPU intensive, so make sure you have the kit for crysis 2. If I was to buy any of them, I would go crysis
I have never played any of the portal games, but i have played Crysis 2. It is not nearly as hard on your PC as the first one.
The campaign is OK. The multiplayer got boring quickly, but i only played the demo on the MP. I can't say which game is better but i can defiantly say that Crysis 2 is NOT worth the 60$ pricetag.