Photoshop is the obvious leader -- it's the industry standard. If you do graphic arts professionally, for both print and for web, it's what I would recommend. It does far more than Paint Shop Pro could ever hope to do.

HOWEVER, if you don't do a whole lot of graphic arts, or if you do it only for a hobby, and if cost is a factor, you should be able to do fine with just Paint Shop Pro. In fact, you should work with what you have, and then if you find yourself limited, then and only then should you consider an alternative to what you already have. There are a lot of people who shell out the big bucks for Photoshop and then not even use it because it's too intimidating for them ... or they end up using only the very basic features which are also available on cheaper graphics programs like PSP. It would be a big waste of money unless you actually plan to use it, and I mean really use it.