Depends on your front setup. If you're running a 1x9 or 2x9, then a medium cage is fine. I'd stay with a long cage for a 3x9. Long cage are much more available and will cover all bases. I like a medium cage for urban stuff, but that's because the derailleur is pretty exposed to grinds. Most bikes have long cage, by far. Of course, if you chg to a long cage you'll likely need a new chain.

Rg.