jQuery 1.4 vs YUI3,
which to use and why?
I like jQuery 1.4: clean, concise, chainable, includes pretty much everything I've ever needed, has a huge library for pretty much everything else.
I think it's a great library, small, compact, etc...
YUI is a great library, it offers a ton, but when I want to include things on a page or access a widget, I just have a namespace 20 characters long in some cases.
Notice how many times those statements include the word I...that's because it works for me. You need to do yourself a service and investigate both (and the other frameworks for that matter) and see what syntax you prefer, which one feels better to you (and/or your team if you're in that situation).