i haven't used one on a bike yet, but have on old ATC's, and i like that your grip's not moving around. you're not constantly twisting, and you can maintain a firm hold on your bike.
in emergency situations, like running into cars, my first reaction is to clamp down on my grips, and with a twist throttle, you could still be fully on the gas.
with a thumb throttle, you'd take your thumb off, and grab onto the grip.
the first (and so far, only) time i hit a car that turned in front of me, i was still full throttle as i banked offa her car. i didn't crash, but i was still on the gas the whole time.
iit's all up to whatever feels right to you.