DS3 or ATM? Which is better?

A client of ours were talked into a 20Meg ATM instead of the 45Meg DS3 that we suggested to them. They are now being told by thier provider that they are utilizing 60% of the ATM (our monitoring says about 25%), and they have been experiencing packet losses and slowdowns ever since.

The link is used SOLELY for Website and Database. No Phone over IP or any of that.

Am I off base here in pointing my finger at the ATM as being the problem? What’s the difference between the two and what can I do/say to get our client convinced that they made the wrong decision? White Papers would be great… This is a dire problem…