I would rather prefer a beta so that we can help Crytek as much as possible to polish the game and get rid of all the bugs that they have not discovered yet. Plus you can find screen shots of Crysis 2(Xbox 360) and an online control scheme without needing to play the demo. A demo would be good but is nothing more than to see wether or not you like the game, and if you like it go buy it. Plus Crysis 2 is going to be far better on the PC for sure, you will have access to the cryengine, mods, more advanced graphics, and the ability to use pretty much any controler you wish. If your PC can play the first Crysis its going to play Crysis 2 better since the minimum specs have been lowered.
But remember, this decision is entirely up to you. Im just giving you a heads up.