Should I learn GTK+ or GTKMM?

I could be accused of bias since I do help contribute to gtkmm, but I was a user first, so... In any case, I would highly recommend gtkmm if you're comfortable with C++. Memory management is much easier with gtkmm than with GTK+ because reference-counted objects are managed automatically with smart pointers. You can also instantiate objects as auto variables (e.g. on the stack) and have their lifetime determined by their scope. So in practice, it's much easier to avoid memory leaks with gtkmm than with GTK+.

Another huge advantage of gtkmm over GTK+ (in my opinion) is the use of a type-safe signals framework. In GTK+, you constantly need to pass things as void pointers and then cast them around to the type you think they should be. In gtkmm, you dont need to do this, and can take advantage of the compiler enforcing type-safety on your signal handlers.

Another big advantage over C/GTK+ is the ease of deriving new classes. In GTK+, you need to write a lot of boilerplate code and basically re-implement things that you get for free in C++ as part of the language (e.g. inheritance, constructors, destructors, etc). This is more tedious and error-prone.

greyfade mentioned that gtkmm is incomplete, and he's right to a certain extent -- gtkmm does not cover absolutely everything in the GTK+ API (though it gets awfully close). But in practice this is not a problem because you can always use the C/GTK+ API directly from your gtkmm code. This C compatibility is a huge advantage of C++ over something like C# or python bindings where you would have no alternatives if the binding didn't cover part of the API.

The only real reasons to choose GTK+ over gtkmm (IMO) are that gtkmm has a little additional overhead since it is a wrapper on top of the C library (but this is generally just a single function call, which is going to have negligible impact), or if you hate or can't use C++.

If you're a C# programmer, why don't you take a look at Vala?

I use pygtk for most of my Linux GUI applications, but Python was simply too slow for the project I'm working on right now, so I was trying to pick one of GTK+ and GTKmm. Then I met Vala.

It's a pretty new language, and therefore documentation is pretty limited at the moment, but I think it has the best of both worlds: C# syntax with C speed.