WHMCS all the way. Most don't think about ease of use for the customers, and WHMCS has it down.
You also have to consider who will be the front runner down the line. It is a hassle to migrate accounts from one system to another. Think of the HDDVD vs. BlueRay a couple years ago. How many people have worthless HDDVDs laying around, and had to repurchase their videos?
It's hard to say try them both, but if not look at the support forums, check screenshots and demos...you'll find what is best for you.