I've used the Aerostich heated grip kit that has the resistor and the Symtec heated grip kit that has the dual circuits and the different elements for the throttle sleeve and bare bar.
The Aerostich kit puts out much more heat, especially if they're wired for hi-lo circuits. If you ride in sub freezing temperatures, this is the kit to use. This kit will put heat into your no matter what type of hand grips you use.
The Symtec kit is a little more elegant. However, in cold conditions, produces no usable heat on the low setting. You have to run it on high to feel anything. If you use heavy rubber or foam grips, no heat for you. And if it's seriously cold, this kit is for humor only.
I have installed these. Very easy to install. Be sure to insulate the left bar so that the bar does not act like a heatsink and suck all of the heat from the element before it can get to the grip. I used duct tape as an insulator and it works great.