Universal thumb throttles are the most versatile throttle installing on either the right or the left and are the most popular. They also work well on trikes and drop bars using Handle Bob mounts.
Left and right wrist throttles are preferred by some, especially motorcycle riders. They come with ergonomic grips with palm rests. (They sometimes require modifications to rapid shifters so that they clear the throttle.)
Left and right thumb throttles extend down and have a natural horizontal thumb action. They have excellent throttle modulation.