A hub motor fits inside the wheel. A belt implies your motor has a sprocket on it that drives a belt and then the wheel also has a sprocket on it that is driven by the belt.
A hub motor of any kind is a fixed quantity. Whatever it's KV is will determine it's maximum RPM at whatever your pack voltage is. That's all the faster the wheels will turn. The only "gearing" you have is larger or smaller diameter tires. A smaller tire will mean a bit more torque and a larger tire means a bit more top speed. There's not exactly a large selection of skateboard hub motors.
With belt drive and probably an outrunner, you can change the gear ratios to get more torque or more top speed...within limits of course. You can also use different sized tires, Outrunner motors are abundant in many sizes and Kv's from dozens of manufacturers.
A hub in wheel design is simple and clean since there is no need for a belt and gearing, but it's also pretty much a fixed quantity with little room to make it different or better. With outrunners and gearing, you can swap out the motors and gearing easily, if you want more speed or power or a bigger motor. None of that is as possible with a hub since the selection of hubs for skate boards is really limited.