In fact, there is only one choice, and Objective-C and Xcode, something else, and you will run into problems and / or limitations.
As already mentioned, Python is one suggestion, but which GUI tool to use? Then packaging becomes a problem.
Mono is fine, but still a bit buggy (and slooooow) on the Mac.
I have not tried Java, but the Apple port on the Java virtual machine is just out of date, do as you want.
Xcode is very good and works well with Mac / iPhone / iPad, etc., but Obj-C takes time to find out what is happening in the background of C # / Java, plus Xcode forces you to use MVC templates in everything that can again to be a cultural shock.
I would say go with Obj-C and Xcode and learn something new.