I could never get used to the thumb throttle after riding motorcycles for 30 years and it was one of the first things i changed when i got the quad. My hand just throbed after riding awhile.
I would think a person would have more control with a twist since you have the entire hand around it. I race MX on dirt bikes for a long time and some of the obsticles may cause you to be thrown around and with the thumb throttle you need to have your thumb on the lever to use it so you may lose contact with it.With a twist you will always be on it.
I suppose it comes down to your preference.
PS- i gave a thumb throttle to a guy at work who hated twist throttles so much , he wanted to try and install it on his dirt bike. Never did hear how that worked out !