I use CDG with authorize.net as the gateway and haven't had any issues with them. Either way you go, you'll want to make sure the gateway they offer is compatible with your billing systems -- for automatic credit card transactions.
-Steven | u2-web, LLC - Clustered Shared Hosting "It is the mark of an educated mind to be able to entertain a thought without accepting it" -Aristotle
So far, UnitedBankService is the least expensive. It is also the most upfront with its fees and has been quick answering sales questions.
CDGCommerce is next in line in terms of support (not cost though), but they do not like my business model (non-hosting).
MerchantPlus is the second least expensive, but do not yet have experience with their support.
FreeAuthNet is next in terms of expense. I worry about their support though because when I contacted them last week they were slow to respond.
Based on research right now, UnitedBankService and FreeAuthNet offer very favorable terms: no contract, no exclusivity clause, no annual fee, no setup fee, low monthly fees, low or no minimums, low fixed transaction cost, and low discount rates.
On paper, United Bank Service is probably the best. If anyone knows of any merchant providers that have similar features let me know!