Not to knock Photoshop, but GIMP does have a few nice features that PhotoShop does not. Namely support for full color brushes. I also don't think PS supports what GIMP calls Hoses/Pipes/etc.

Just in case it does not or you don't know what that means (and PS does support it), it's a way to make a layered brush and each "draw" of the brush chooses a random layer to output. Think of having 10, 20, 30, whatever different mountains/forests/etc in a single brush and just swiping your mouse to "paint" and having a variety of shapes show up on your map.

Also, from what I hear, Photoshop's build in cloud renderer is not very configurable and GIMP's is. To be fair here, while GIMP's build in clouds is good, it's no where near great. Both have third party plugins for cloud generation that do a much better job than the native one, but while I can speak to the one in GIMP as being free, I don't know if any of the PS ones are free and offer the same level of customization.

On PS's side, there are much better brush dynamics(though GIMP 2.8 goes a nice way toward decreasing this gap) and the BIG thing is adjustment layers, CMYK, and >8bit color depth support.

In the end, it depends on what you really need. Also, as Beoner says, there is nothing stopping you from using one or the other or even both on a single project(well, except for having to be careful with file formats and the merging of layers that may result from that process.)