I've never played either of those games, but I definitely would have jumped on Halo 3 if I didn't already own it! I guess I could always download it and then sell my CD...
I bought A world of Keflings a couple years ago. It's fun and addictive. Also bought Iron Brigade and was OK. Wish is was a AAA title....or at least one of them.
I'm surprised that some people never played Halo 3. I'll never trade that game in and probably try and play it years down the road when the Xbox One is the new everyday console