The Stock Kymco drive belt is just fine, but if you want to "go the extra mile" then you could get a Kevlar belt. Kevlar belts are not indestructible though. Personally, even with a Kevlar belt, I would still replace the belt at the manufacturer's recommended intervals. It's better to do it that way than to end up stranded, pushing the bike or waiting for a ride in the cold.
Having said that, if you're replacing the belt when it gets to be high millage, you may as well just stick with the stock belt anyway, because you'll just be spending more money. I don't think Kevlar is that much more reliable than stock. Perhaps check prices of stock vs Kevlar, you might find an after market one cheaper for all I know.
~Josh