I prefer starting horse and human students off with clickers. Eventually we may move to a tongue pop or something else, but I think the clarity of the clicker is powerful. It is also the same between people which is important for horses that are handled by more than one person, like in a lesson program. I usually start people off with the button clickers (EcoCity) as they're easier to press and ergonomically fitted to the hand so easier to hold for kids. However, they're a bit quiet for distance work, so I prefer using box clickers (Patty Both), for myself and more advanced students.
I just recently discovered some affordable ring clickers by PetSafe and ordered a bunch. I think they'll be much easier for kids, more convenient for a lot of adults, easier when riding, etc.