Here are my two cents on the subject.
PS and PsP are both expensive, but I use PhotoShop at school. It is a great program, and I would recommend it but for several reasons:
1. Cost. Why shell out money for a program that you are using for what is essentially a hobby? (In the case of Civ Modding). Besides, with a program that you purchase, once it becomes obsolete, you will either have to buy the new program or pay for an upgrade. With open-source, you simply download the next version and Voila! you can continue working.
2. How many features do you really need? Granted, PS has more features (i.e. more extensions, filters, etc.), but using a combination of the GIMP's filters and extensions, you can pretty much replicate anything you can do in PS. Plus GIMP has a few things that PS doesn't (softglow, etc.).
3. I have been working with GIMP for nigh on two years now, and I have always seen it as an easy program to learn.
I say, if you are going to use this program for things other (and more important) than Civ, then use PS or PsP. Otherwise, try out GIMP. If you don't like, you have lost nothing, whereas if you purchase a program or suite of programs, you may be disappointed and be out anywhere from 85-700 bucks.