Yeah, Bad Company 2 is the best popular shooter out now. Just Cause 2 was kinda fun in the demo, but got old fast and didn't really flow that well. It tried to do too much and didn't get the important things to work as well as they should. Bad Company 2 has a fun (and quite hilarious) campaign, and the best online FPS gameplay currently available. BFBC2 all the way.
Unbiased opinion here, I have both. As Rogue pointed out, BC2 has the best on line multi player ATM and a very fun and interesting campaign. JC2 has no multiplayer plans as pointed out on kotaku and the campaign is meh. It's a lot like GTA but more repetitive. I enjoy just doing stunts etc and blowing things up. Travelling is kind of annoying, I think they just couldn't be arsed to make a realistic vehicle that travels fast enough to get point to point without having to wait 5 minutes. I think they just blew it with the teleport. There you go. IMO, if you want a good game overall, BC2 and if you want something for when your bored just to mess around on, then JC2.
Dude, Just Cause 2 sucks! i played the demo and hated it. I dunno, maybe you might like it, but I would highly recommend getting BC2. Its well worth the money, and you should get it while the limited editions are still here, so you don't have to pay 1200 MS points for it.