Both programs are for different things
Aperture 2 is basically a photo browser, with basic manipulation to images, like Brightness, contrast, reduce highlights, shadows.
Photoshop is a program for editing those pictures more indepth. You have your layers, your brushes which more carefully edit the picture.
I have both and use both at the same time
If i can't do something in aperture i bring it up in photoshop and work with it there and then save back to aperture where you can print, slideshow, books and make websites.
just got aperture 2 and it's quite fast, lacking a few features that lightroom has but probably because i just got aperture and still exploring!