As far as program quality, Photoshop obviously. Gimp is opensource and photoshop cost an arm and leg.
If you are new to both programs though, your skill level will be the same. If i were you i'd start out on gimp, if you then seriously got into art buy either photoshop or a program like it such as coral painter, artrage, sai paint, etc.
But as i said above, the program is just a tool, the quality of your art is ultimately up to you and your skill level.