i have run the ws7 II, III, and c6bbt. Not sure what you mean by ws7t? I ran the base model of the extech .21. I broke it in and was inpressed by the low temps, power throught out the rpm range, and solid runtimes(not the greatest runtimes but I was running the jp-3 pipe and getting 8.5 - 9 minutes. I hear with the trinity pipe you get more power and over 10 minutes a tank.) The Rb engines were great as well but definately a little more money. The main difference is customer support. For me if I had a problem with my Extech and posted on this forum chad says send it to me I'll take care of it with a fast turn around. If I ever had a problem with my RB or have read on the RB forum about people having problems I don't see much coperation from RB. There are a lot of racers that try to help, but thats about it. You can't go wrong with the quality and performance of either engine, but the Exech customer support is nice. Im not tring to say Rb has no customer support but it does not compare to Trinity