I would suggest looking at your customer base first.
You could perhaps set up your site using PayPal as a payment method since it's the cheaper alternative. Then gather some kind of data/metrics from your logs to determine if users turn away when they hit your payment page. Though this might be because of other factors (price of product), you'll have a general idea. Based on this, you can get a better understanding of your customer base and then decide if going the payment gateway route will benefit you. You'll obviously have to factor in cost of the gateway and all that jazz.
You may even consider factoring in your customer base average age if you have this information... I don't have concrete evidence, but it would seem from past experience that the older generations would rather prefer the standard merchant processor over PayPal.
As for my preference, I would prefer to have PayPal as an option when I buy something. As far as testing goes for PayPal, it's pretty straight forward. They provide a "Sandbox" which allows you to test almost all the functionality that you would encounter if the code was on a production server.