Telstar,
Adobe's Photoshop and Apple's Aperture are not really comparable programs. The comparison between Adobe's and Apple's programs should be Adobe Lightroom and Apple's Aperture.
Aperture and Lightroom are programs with organizational
and image editing features with very god RAW file conversion abilities. The image editing capabilities are more of an overall image adjustment type (With some cloning features in Lightroom).
If you need more sophisticated image editing such as layers, masks, selections and so on, then Photoshop, (CS2 as its current version is called) or Photoshop Elements is the program.
You can try Aperture for 30 days and I believe Lightroom is also available for trial.