You would be well-served to use Qt and Cocoa enough to get familiar with them. You may or may not like the language, but the organization and consistency of both are excellent. In particular, QT's signals/slots, naming consistency, and layouts make writing UIs easy without looking at the documentation (once you've used it a bit). Cocoa is well thought out, but a bit dated. (UIKit is not so well thought out, and forces MVC on you, which is often unnecessary and unhelpful, and then it completely botches UITableView by muddling up the M, V, and C. Qt's QTableView is much saner.) Unfortunately, it's also hard to write a UI in code in Cocoa, since you have to write the positioning system yourself due to no layouts. How they handle different screen resolutions is brilliant, though.

You might also want your UI library to be cross-platform, which will limit your choice of languages, especially if you ever want to port to mobile.