Photoshop is more powerful, I use Fireworks, and sometimes I come up against a limitation in the software where I say, I wish I had Photoshop...but Fireworks is easier to use. All the cool graphic plugins in the world are made for Photoshop.
Sure you can, specially when there is no way to guess what is better or worst and because I still do not know how to use completely any package. I was just playing with the filters to make that.
But there are lots of things GIMP can do and Photoshop no. Like using the GIMPresionist filter :P.