Renntec are the only ones worth having really. They are big, heavy and strong. Yes they limit corner clearance, and they add weight, and don't play well with belly pans. I had some for a while and they are handy for chaining the bike up with!
There are some other styles that only mount in two places (the front two as per Renntec) but there is a danger of twisting the frame with these.
Downpipes: motad are common, I've got lazer on mine. Both are good.