Belts are marginally more efficient than chain (and I say marginally because #25 chain is some pretty good stuff). As noted above, it comes down to a few percentage points. However, belt requires 75 times more tension for proper tooth interactions than equivalent chain. But don't take my word on it, take a gander over at
http://chain-guide.com/
As for personal experience, I can say that if your chassis isn't bone solid rigid, you'll have issues with maintaining belt tension during use. Our tower caused our C-Channel chassis to twist (because open sided tubes like C-channel carry significantly less torque than closed square tubing) and maintaining tension from our gearbox to our wheels was an effort in futility.