I do NOT like gimp. Not because of the interface, I don't like the way it interprets movement on a tablet. Really, the best I can do to explain that is to say that Photoshop feels like working on paper with whatever tool you chose as a brush, Gimp is always like drawing on coated paper with a permanent marker. It feels really slick to me, and I dislike all of the default brushes. Though you can download brushes and addons and stuff to make it, I don't know if it's better but it would be different. I've never really bothered with it, I love photoshop.
Photoshop is not free, but you can download a trial.
But if you find gimp confusing, photoshop will be way worse. It's got lots of capability, but you're going to have to either play or read a lot to get good at using it.
I find Muro- the dA art program- to be superior to gimp in many ways- the only thing I dislike is that you can't put layers UNDER an existing one. Which is really just a matter of a little planning before you start to work.
Photoshop isn't free. You can get a trial version like other people have said. It's very expensive though. Photoshop like GIMP, is just as difficult to learn if you haven't touched anything more than MS Paint. You do the exact same things in GIMP as you do in Photoshop. There are differences on how you go about them but its the same thing. You will have to learn how to use those types of programs. It isn't going to be easy and may take some time. My only suggestion is try to find as many tutorials you can on how to GIMP. You will be doing the exact same thing if you use Photoshop. I use both and once you know what to do in one, you can mimic it in another.
GIMP has one hell of a learning curve. It's going to take a while to master.
Photoshop is considerably easier to grasp, but as noted, it is not free. You can get a trial version, but that will only last for a month, and then in order to get another trial you'll have to either get it on another computer or reinstall your OS on the one you're using now.
Believe me, though, GIMP can do almost anything Photoshop can. You just have to know what you're doing. And maybe have some patience, because GIMP is relatively laggy even on computers built for huge art projects.
My knowledge of GIMP comes purely from personal experience (been working with the program off-and-on for about five years now), but I'm sure there are plenty of useful tutorials you can find even right here on dA.