I've got the SDP kit from Scott, fit and finish was great and the pipe is SS too, not aluminum...it fit the stock location perfect and comes with a welded on brace...I wound up cutting off the flange and brace once I upgraded to a 1pc Y-bridge tho...
I like that he offers custom colors for his kits and can accommodate in a timely manner.
Although I'd like to see what Tony's (DHD) "super big air" kit is like...it's a little more "complete" of a kit, I believe it comes with a Y-bridge, cold side and a hot side...but I think it's all aluminum...I think HSP's stuff is all aluminum too, at least the 3" hot side I got from them was, I would have preferred SS
I like Scott's design the best from what I've seen, IMO it's more versatile