What is the difference between simulating an engine failure by pulling the mixture or the throttle?!
I always used the mixture - a bit less stress on the carby and induction bits (as the throttle buttlerfly remains open), and the chances of the mixture getting stuck in the off position are the same as the throttle getting stuck.
(In fact one of my company's students had a stuck throttle on first area solo, and performed a textbook PFL into a clear paddock with no damage to aircraft or student.)