Braintree is awesome with Rails apps (and particularly rock solid if you needed to implement recurring charges). The only issue is you would need to process $1+ mm through your site, otherwise, there'd be an additional monthly charge. Highly recommended however.
Also, I'd assume that you're integrating the APIs through ActiveMerchant? I'd definitely use that regardless of which reseller/merchant provider you choose.
Quick question: how much is the additional monthly charge? I've got an application on the back burner I was considering using them for, largely on the recommendation of the guy who wrote the Rails kit for credit card processing.
Also, I'd assume that you're integrating the APIs through ActiveMerchant? I'd definitely use that regardless of which reseller/merchant provider you choose.