I have the the XR10 pro and XR10 pro stock. Both are great esc's. You will notice no difference in them with a 17.5 or 13.5 motor. From what the experts have told me is that there is no advantage to the higher amp esc for stock racing. Your on going to use so much of what it has to offer and the stock one has more than enough for stock racing. I like the XR10 stock for my 2wd stock car for its size and super light weight. I run the XR10 Pro in my 4wd. I have also run the Tekin ESCs including the RS Pro. I prefer the Hobbywing stuff. The brakes and overall feel to the Hobbywing suit my driving style. The Tekins may have a lower resistance but you will never notice it. The top ESC companies all have pretty low resistance and it is almost like splitting hairs now.