This leads to my question. What are the differences between how iPhoto and Aperture organize.
I am no a pro, though I do care a great deal about my photographs. I am not really crazy about having one huge library, even if I can break it up into album views as well, but to change libraries also seems a bit of a pain.
Both Aperture and Lightroom will completely manage the library for you, a la iPhoto, if you wish (and by default). You can also choose to keep your files organized yourself, and just tell Aperture or Lightroom where to find them.
I don't see a disadvantage to letting Aperture (or Lightroom) manage your library for you, except maybe in the case of professionals that have huge number of photos - meaning they've got too many photos to physically fit on a single disk. Both applications can handle this scenario though.
Aperture has a built-in backup system called "vaults" that lets you keep backups of your library in multiple places, if you wish. Vaults are trivially easy to use. I haven't tried Lightroom's backup tool - I assume it has one.
As far as how the image handling
feels inside the applications: When I was trying Lightroom, its photo management felt very much like Adobe Bridge to me. Some people will like that because they are familiar with it; but I never really liked Bridge. Aperture's photo management just seems to stay out of my way, and lets me work. With Lightroom it seems (to me) you need to be much more aware of what module you are in, or want to be in.
Really, though, to beat on the same drum as my other post - trial versions of both are free, so the best solution is to check both out for yourself.