I like the brake lever option. The shifter is designed to stay in the place that you put it. Where that is super convenient I am worried if there were any situation that you would want a "kill switch?" It's there when you need it and off when you let go, no worries on looking down to figure out where it needs to be set. Rock ShoX has a pop-lock shifter(very compact) option that I think I will install that has a on/off option for a suspension lockout that would be nice. You set it at your desired speed and then tap it to release.
I'm looking at this at a Bike tech point of view. and am more than willing to help with the set up if anyone wants. I have a good handle on all the products, Just haven't used the hand throttle much yet.
The shifter that I've seen in the Offroad market are cheap, like a few dollars and clunky. Why not throw some cool gadgets at your rig-make it worth the effort.