I put the Terry Components Terminal Velocity II on my 03 SERK at the same time I did the S&S570 cams. I went with it mainly because it's much less money than the others, around $400 from Directparts.com. Had to weld in a bung in the rear pipe and solder the wires into the harness, which is no big deal. My bike's out of warranty anyway and I plan to keep it many years, so that's no problem for me.
My bike has an SE air cleaner and Rinehart true duals with no factory downloads. After about 300 + miles the bike runs great. At first it was a little rough but kept getting better each time I rode. It was running very well within 50 miles of riding, but got better as time went on. Based on my experiences so far, I'd do it again in a minute. Like I said, it's $3-400 less than the other alternatives and on my bike seems to work very well. I'm sure a dual sensor might work a little better, but I guess it depends on what you want out of it and how much you can spend. For me, this does the job at a lower cost than the others.