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!
If you have Photoshop, you dont need either Aperture or Lightroom except for the file management. for me, I prefer to manage my files on a stand alone image viewer, edit the files on a image editor. less confusing for me.
that said, Aperture or Lightroom is targeted at photographer, for touching up the photo. where as Photopshop is the industrial defacto image editor--no contest.
Aperture and Lightroom are very good for file management plus some pretty good editing. PS does not do file management.
Myself I use Lightroom 95% of the time and when needed then I go into PS.
Try the demos.