Shopsite compares VERY well with all the defunct carts that appeared over the years. :)
Nowadays the main concern shouldn't be the feature set but whether the cart company will survive these tough times. Shopsite is regularly enhanced and its rare bugs are patched within days of discovery. The user group is large and vibrant. There is almost no downtime. Shopsite is located in Utah. The company is not publicly owned, which is a major plus in my opinion.
How about Yahoo stores? Publicly owned, but a cart with a great track record too.
What are your client's specific requirements?