My son and I own quads with both types of throttle.
For trails and hills where you have to use arms to hang on the thumb throttle shines.
For flat track and speedway where the body is nearly always off the side of the bike, the twist throttle seems much easier to control.
As for Moto cross, I come from a background of two wheelers and prefer the twist throttle, (old habits die hard).
My 14yr old son is in transition with a twist throttle fitted just last week. He says that he likes it, but I have observed the nose of the Blaster a little high coming off of the big jumps.
Word of warning If you change over to a twist throttle, the first time or two you feed it the herbs can take you by surprise.
With a thumb throttle when you give it heaps you have to make a conscious effort to maintain pressure on the lever.
With a twister, the more you give it, the more your body wants to slide to the back of the seat and can cause the wrist to give even more go juice.
I find a way to help unwanted throttle rotation on acceleration is to place two fingers on the front brake lever.
Both throttle configurations have their goods and bad's, try a twister you may like it.