I prefer flat if I am doing a small cook (picks up all the seasoning from my grates), but the quality does not suffer using a rack. Sometimes, I just use the rack (and cut a couple of slabs in half to fill the rack) just so I can put a pan underneath and minimize the mess. I use the Weber original.
http://www.amazon.com/Weber-6469-Ori.../dp/B005LR6Z56