Here is a few good sites that may help, first is a blog that compares a lot of carts and talks about pros and cons of each the other 2 sites review carts.
good luck.
on this make sure you read down into the replies, much more info on lots of carts.
scriptygoddess » Blog Archive » Cube Cart vs. Zen Cart Research over 200 shopping carts at Shopping Cart Reviews Shopping Cart Review 2008