I've driven all types of popular TC, but I keep coming back to a belt drive car, which is the XXX-S. It's drivetrain is sealed for the most part, so it keeps out the big stuff, like stones. It still allows a little dust, unless you take the time to seal the lexan gear guard and bottom belt cover with some silicon sealant or something like that. Also, the chassis is tweak free, and doesn't seem to want to tweak itself when abused/crashed hard. I purchased one of the first losi's to hit the LHS shelves, along with the first run of graphite chassis. To this day I'm still driving with the same chassis, and the thing is perfectly true and tweak free after years of use/abuse. (I just checked it last night!) Although I do have a G+ chassis waiting in the wings.
Plus and this is just my opinion and my own unique driving style, but the Losi just works for me. I've never driven a car that just flowed with my driving style like the losi does. It's why, no matter how many TCs I own , trade, or sell, the losi never leaves my stable.
But I do agree that the belt tension/skipping thing is a bit of an annoyance. But hey, it's worth it IMO.
About the only difference I notice between belt and shaft is that initial snap from a shaft car when you hit the gas off a corner. A belt car kind of "hesitates" as the belt tensions back up. But as far as hurting your lap, I don't really think it has an effect....again, IMO.