Portal 2 will be superior on PS3.
MS is too stuck up to bring Steamworks to XBL. That's their loss.
Valve didn't like PS3 in the past because they didn't like coding for it. The Source engine is very very traditional so it works best with PC architectures. L4D and Half Life are essentially PC games that were directly ported to 360. The console version of L4D1/L4D2 are essentially cash-in ports.
Though, anyone honestly thinking Valve will port a 2 year old game to the PS3 "cuz steam is coming!!" should just STFU. Portal 2 obviously isn't the only game they are going to use steamworks on for PS3, but that does't mean they are going to start porting relatively older games to the PS3 for full price. Maybe a L4D3 or another new IP though...