Charcoal has a better flavor, and give you more diversity. If you get a charcoal, make sure it's deep so you can use it for hardwood and smoking.
Gas is cleaner, easier, and cooks evenly. You can buy flavor pellets for gas now, as well as smoker boxes. I've never used either with gas.
If you are serious about food flavor and cooking, I'd go with charcoal/wood. If you just like a burger or steak on the grill, fast and easy, I'd get gas.