If you buy the PS3 version, you can sync it with your steam account, and get the PC version for free. I also heard there is cross platform (PC and PS3) co-op for Portal 2.
Also, for the US people, you can pick up Portal 2 from Gamestop for $39.99 (or $29.99 for PC), or $34.99 at Kmart. Walmart/Bestbuy (or at least my local ones) also price match ads, so if you don't want to go to Gamestop, then go to store that price matches.