I found that baby backs are best cut in half if I am going to cook them in the CS. They usually are too long to fit the smaller Smokette shelves. If you can live with the half rack approach, I think you easily could get 4 racks on the three shelves, maybe even more. I haven't had to do more than two racks at a time, so I haven't pushed the limit.
Some have suggested rolling the rack into a circle to fit the shelves, but most of my racks aren't that flexible. I like Ricky's advice that you hang the ribs in the larger CS.
If you wife likes the larger unit, I suggest you move quickly if you have the space. Most on the forum have to convince the Mrs. to get the 50/55 - you're lucky to get "pre-approved" for the larger model!
FWIW, I also have some problems fitting a large packer brisket in the smaller CS. If you intend to do many of those, you will probably be happier with the larger unit.
The larger unit also seems to be the preferred choice of those who want to smoke sausage. You can purchase a baffle to keep the temps down and the links can hang easier.