What do you want to do?
What you are planning to do with your images should really guide your choice. As mentioned, Aperture and Adobe's lightroom are not the same as Photo Shop. You can perform some marvelous things to digital images in RAW format with Aperture as well as catalog your work. The enhancement ability is awesome. However, image adjustments are all you can do. If your intent is to add/remove elements from your image, or any other drastic image modification, PhotoShop is the only way to go. The advice to download the free trials is excellent. The best way to make up your mind is to poke around in each to see what their respective pluses and minuses are. I have both Aperture and PhotoShop CS3 and find I use Aperture more. Which either way you go, practice as much as you can and never shy away from trying a feature. But, always remember, the BEST digital image is the one the "see" before taking the shot. No amount of post processing can make up for a poorly thought out shot. Work on producing images that do not require a lot of post processing.