I used Paint Shop Pro 7, Photoshop 6 and Corel Photo Paint 10 for image editing, while I've tried also Procreate Painter 7 and Metacreations Painter Classic (plus a freeware program called Open Canvas ^^;
for coloring images.
For image editing, I personally prefer Photoshop, because it has more features and despite its complexity, when you get used to it it's more easy to obtain anything you want with it. Adding to it, it's kinda good also for coloring images, while I don't think the same of Paint Shop Pro. For coloring Procreate Painter is the best, I think, but it's completely unuseful for other things, so I wouldn't suggest you to get this expensive software if you're not exclusively looking for coloring tools. Adding to it, also for coloring, Painter it's really really difficult to use. It's a mess if you're not used to it
On the other hand, what artist said is true: Paint Shop Pro is cheaper than Photoshop. I don't know how much you want to spend on this purchase, but Photoshop is not the best choice if you want something cheap.