Well, it kind of depends. The Cocoa API's are generally more geared towards helping you create a user interface, which requires you to be pretty good with object oriented design. That's not exclusivley true, but I think there are plenty of java API's for you to learn before learning the Cocoa API's. If you think you are game, it shouldn't take you very long at all to learn the syntax of java; it's really easy, considering it performs garbage collection for you. There are "this"'s and .stuff() and .stuff and other stuff, but it's all pretty easy after you now OO.
Honestly, I'd learn java first, then learn cocoa. It will be a lot easier if you have the rules of java in general down before you try to learn an API, IMHO.
Just my 2 cents,
F-bacher