Just because it's a GUI doesn't mean you have to use the mouse. I find gvim much faster than vim in the terminal because it doesn't have to print a screen full of glyphs for each update.
On the other hand, vim + tmux is pretty awesome, and for that reason I prefer vim on the terminal. I've never particularly liked gvim, even if I'm not using tmux. In Linux, hitting the start or end of a buffer causes some sort of screen redraw that I've never been able to fix, but that might have to do with my configuration.
