Aperture or Lightroom for editing software

Between Aperture and Lightroom, IMHO that is a no-brainer: Aperture. It is less expensive and an easy transition from iPhoto (which you are already using). On top of that, I find Aperture superior to Lightroom in almost every aspect (but not all!). That said, there are some differences in all this software:

iPhoto: entry level image management, very basic editing.

Aperture/Lightroom: extensive image management, more powerful editing tools

Photoshop: very powerful image editing, NO image management.

Photoshop Elements: Elements is two programs in one... The Organizer and the Editor.The Organizer is the image management tool and the Editor is the editing too. PSE will do most of what Photoshop will do, but of course not all. As example, Elements only works with 8-bit files, not 16-bit files. ​

With all of that in mind, in most ​professional ​workflows, you have both: an image ​management​ tool AND an image ​e​editing​ tool. Now Aperture (recommended) OR Lightroom certainly meets the image management needs and it is possible that either can meet your editing needs. Both will be very, very similar in their "editing" powers. If you find that the editing capabilities of either app isn't meeting your needs then you will need either Photoshop or Photoshop Elements. Elements is indeed difficult to beat as you can often find it on sale for like $50!