It depends. It really, really depends. Portal 2 is my Game of the Year for 2011 so far. I loved every minute of it. It was smart, funny, and all around an enjoyable experience. However, LA Noire has a lot more substance to it. Portal 2 was a blast, but it was simple. LA Noire is like a mixture between Red Dead Redemption, Phoenix Wright, and Indigo Prophecy and it's also nearly 20 hours long and that's if you don't screw around in the sandbox (Which you can. You can just drive around for hours... the map is massive). Portal 2 was also about 6 hours long with hardly any replay value and it was a lot more simple.
The thing is... I enjoyed Portal 2 more than I'm enjoying LA Noire, but I think LA Noire is all around a better game. If I were you, I'd just save up all my money for inFamous 2.