This comes down less to fitness and more to what kind of day you want.
You want to set your own pace, stop wherever you like, and don't mind carrying your own gear. Trekking also reaches a few places horses can't — the final stretch to Ala-Kul lake, for instance, is foot-only.
You want to cover more ground with less physical effort, you're traveling with kids or older family members, or you simply enjoy riding as part of the experience rather than a means to an end. It's also the more comfortable option for multi-day routes across open pasture, like Song-Kul or Kol-Ukok.
Not sure which fits your group? Tell us your fitness level and who's traveling with you, and we'll point you at the right tour.