I find that Gimp is a little worse than some other programs at memory management. What size canvases are you working with? What machine are you using? I find that Gimp can run A4 at 300dpi with 15 layers or so on 1Gb or Ram before it starts slowing down. I have to borrow other computers for anything larger. Photoshop will be better at this I guess because it's optimised for professional use. That's my guess anyway. If its slow and crashy with sizes lower than that (or larger with a heftier computer - which I assume you have since you're running some pretty resource hungry programs) then there might be something up with the settings.

I like it better than photoshop, but that's just a matter of personal experience. I'm sure that if I'd spent a year with photoshop I'd be pretty happy using it over Gimp.