I was thinking of doing a project using the Closure library, but I guess I'll stick with jQuery

Might I suggest taking a look at Dojo as well, depending on your requirements, Dojo is really good at it's target. Which is large browser based apps. I use jQuery extensively for the smaller problem sets, but their is no substitute for Dojo when you start getting into large browser apps. Dojo provides the full stack for browser app developers and would be similar to Closure in the segment that it targets. It's worth s look to see if you like it, further it can be used in conjunction with jQuery they are not incompatible and play very well together.