How to choose your payment gateway
Here is a simple calculation I like to use for most of my business dealings.
(Support-Hassle)/Cost+Setup cost vs Profit
What I look at first is the cost of the service. Very important with payment gateways to look at the monthly fee and the transaction fee. Everyone is a little different and some will suit your sweet profit spot better than others. If you are already up and running the calculation is easy...e.g. All cost of current provider vs the new providers fees.
If you are just setting up you'll need to have an educated guess. If you are just testing the waters don't commit yourself to 3 years of payments.
Other things to consider
Who pays for the setup cost. Make sure you talk to your developer about this before you commit. Once in a while I'll have an excited client call in to tell me they about to sign up on a great payment gateway deal, only to have me explain the last one we setup cost around a $1000 and the ongoing support was zero and updates were random and without notification. I can't say how important this is. Banks and smaller operators are notorious for promising everything and then providing nothing after you sign your life away.
Do they provide on going support and what hours is the support available. The thing with e-commerce and websites is they often are 24hr operations and the last thing you want is your website payment gate way down in the middle of the night and there is no one to call until 9am the next day. Even worse is finding out the support is in another country in another time zone. So every time you call is out of hours and costing dollars for off shore calls.
Do they provide support to your developer. This can be worth hundreds of dollars. I know some of our regular Gateway providers give us access to a team of there developers for projects at no cost. We can use them to help trouble shoot, develop new service for clients. The value is priceless.
Will they help you with PCI Compliance. This is very important. Every Website recording user details or taking money should have a SSL Encyrption Cerificate. However the liablity if there is a breach can fall solely on your shoulders if you are hosting the gateway or the providers if they are giving you a hosted gateway.