I think technique wise, the standing up bicep curl form is easier because you have good sight of the instructor, whereas on your back laying on the step doing the tricep track you really have 2 options, 1 to look at the instructor to make sure your doing it at the right speed or 2 just listening to the instructors cues and keep your sight focused on your tricep technique.
Even though I've done about 20 pump classes, during tricep tracks on my back, I have nfi what the instructors are saying most of the time, calling out numbers which don't seem to mean anything.. so my vision is usually kept on the instructor and which results in my tricep exercise technique to be very bad.