I agree with Twirlyjoy - get the trial versions of each of the programs and try them out. Mind you Photoshop will be a little more daunting until you actually do the tutorials and take some classes but the classes are sooooo worth it!.. And there is the cost consideration too. It all boils down to what you want, like and are willing to spend.
I have PhotoShop CS3 and after taking the classes here LOVE IT. It's so much more powerful than I expected so I'm glad I went with it. I don't know what the capabilities of the other programs are though as this program was a gift so I didn't get to try the others out first