Trials 2 is one of the most addictive games I've ever played, and also one of the most frustrating and stress inducing. Basically you're on a motorbike and the levels are short stunt based levels, all you have to do is get through them. It's technically a platformer though. A lot of levels have you climbing stacks of tires or jumping from ledge to ledge. It's physics based and you can shift weight on the bike to make it jump higher or stay lower. It's a ton of fun. As far as Monster Trucks Nitro I've only played the iphone version which is a bit short, so I cant say anything about the PC version.
I would say that both games for $4.50 is a steal (Trials 2 is easily worth that on it's own)