They're two entirely different apps with very different purposes/workflows.
Aperture is the equivalent of Adobe Lightroom - it allows you to maintain a photo library and do basic manipulations (think of it as a digital darkroom).
Photoshop is a whole other animal - it doesn't have the library management functions but allows you to alter and re-touch photographs to a much more significant degree - it can, however, be something of a sledgehammer to crack a nut, and for adjustment of exposure on RAW images, for instance, it's often simpler to do the job in Aperture or Lightroom.
Most pros would own both, but it really depends on what you need.