First, I don't own a Mac. In my case the solution was a given. Gimp was free Open source and PS cost several hundred dollars. I'm not a pirate and my budget was pretty close to zero so I chose Gimp.

Will I someday get Photoshop? Perhaps but it will have to be due to client demands or it allowing me to be more productive. Since I tend to hand paint stuff I don't think that will be the case. For me getting a tablet will be more beneficial.

If the cost was not a part of the equation I would probably get PS just because when you look around at professional artists there seem to be more of them using it.