If you want to do heavy image editing and post processing Aperture might be a good choice for you
Aperture gives me more editing possibilities.
My first Mac encounter with image editing and organization program was iPhoto. It became used a lot and many of the features suited me. The way we could edit our images without loosing the original file. If we wanted we just told iPhoto we wanted our original back and all the changes we made disappeared. Until I got my DSLR camera that is.
When taking pictures in RAW format suddenly you get more possibilities when doing editing, and for this Apple Aperture suited me better. Now I adjust more on a detail level compared to iPhoto and that is the main reason this is the application I use today. Also with the organization and the good vault backup feature I feel all my needs are filled.
You will also find the same features as in iPhoto when editing images, the original master file stays intact and the application will specifically ask you if you are about to do something with the original file. Like delete it or something like that. This is one important feature that have saved me on a number of occasions !
If you have one external hard drive it is also easy to create backups of your entire library. There is a feature called vault, where you just set the path to where you want the backup to be placed and the application does the rest. Later you only need to do just one mouse click and the backup process starts. It cant be any easier. This backup can only be read correctly by the application, if you want to make the images available to other applications for later editing you have to select the images you want, and export them.
Download the trial version here and try for yourself.