I posted a week or so back one some research I was doing, alluded to genealogy.com, and asked if there was anyone who had used their census images before. I don't recall anyone using them, so I'm taking the liberty of comparing my experience with genealogy.com and ancestry.com. Note, this is just my opinion, and relates only to the US census collections of these services.

The main difference was clarity of images, and quality of search/index options. In each of these regards, I believe genealogy.com was the hands down winner. With ancestry, I'd experience a good deal of dark or poorly filmed images. I did not have a similar experience with genealogy.com. Au contraire, the images were very well copied and clear (and in a format you could reasonably download and preserve, unlike ancestry). Also, the page navigation was superior at genealogy, imo. The search and index functions seemed more complete at genealogy.com

I only had the genealogy membership for a week, but I did some major searching, so I think my impressions on the indices and images are well founded.