I would recommend you guys go with Cocoa. Objective-C is a neat language, and I really enjoy it. If you want to do visual stuff, then java is right out. (Unless you do java Cocoa). Having to make interfaces with straight java gives me nightmares.
I would recommend "Cocoa Programming for Mac OS X" by Aaron Hillegass. It really helped me get a feel for how to work with Coca and Objective-C. By the time you get done with the book, you've made a lot of small apps that show you how to work with most of the Cocoa framework. If you want to do networking, there is the follow on book which deals with that.
With interface builder and XCode, you can whip up a gui really quick. My girlfriend made a program with a little help. It doesn't do much, you som buttons and different strings come up, but she's proud of saying she made a program.
Just my thoughts.