For me it is always photoshop, I have been using photoshop for about 10 years and am quite familiar with it.
I have not tried Gimp till now so cant say about it.
I haven't ever tried Photoshop so cant compare it to Gimp, but i have used Gimp and liked it. :D I would recommend Gimp to others looking for a good free software.
With GIMP you get great software considering that you don't have to pay a single penny for it, and it's a pretty decent graphics editor all together. Still, I'll have to go with Photoshop on this one. Photoshop is way to expensive, but then again it is a really powerful piece of software where you can do pretty much anything with a picture, and create pretty much anything out of nothing. The UI is also amazing. I've used Photoshop since 2008-09, and I probably will keep doing so for the next 10-20 years.
Even though GIMP is usually described as the free photoshop I have never used it. I use photoshop instead. If I had heard about GIMP before I used photoshop I would have definately tried this. GIMP might be good but with its floating screens, it looks silly.
Photoshop is way better than GIMP but that is because GIMP is a free tool for causal designers and Photoshop is used in the design industry by people all over the world. It's basically the default program to use at this stage. It's been developed over the years whereby it's now very feature rich and you can do so much with the software. It's just a shame it's expensive. Price is where GIMP wins.