I've tried both and have settled with Volusion. It's not perfect and there are things that drive me batty about it but it is solid and a great value. They do charge bandwidth charges in excess of the allotted usage, but that obviously scales with traffic, which should mean more sales. We have no issue with that. Much better than paying a % of sales like with Yahoo.

Americommerce has great ideas, but they just don't deliver. They're better at designing what a great ecommerce solution should be than at delivering a solid, bug-free solution. We tried them out and bailed because we couldn't count on them and couldn't imagine building a store with a platform that was that buggy.