Yes, a shaft is probably stronger than a belt, BUT... say goodbye to your diff gears. I have to replace the gears on my 18b every once in a while. There is no flex in the shaft, so all the stress goes to the diffs.

On a belt driven car, you have one gear on the diff and the belt sits in its track. It's a great setup, the diff gears should last 3 times as long. Although you will hear the belt slip sometimes on takeoffs from a dead stop, which is unpleasant, but it wont or at least a havn't seen it hurt anything. You'd be surprised how much that little bit of flex changes the way it accelerates and handles though. It also sounds a lot smoother.

I do recommend having the belt sealed off. If your racing or driving on a preped track then it doesn't matter but I drive mine on the street in front of my house and a thin layer of dust will get in there. It doesn't hurt anything, I clean it every once in a while. It would probably get ugly if it were not sealed off though. You don't want debris in the belt.