Short should be fine on a hard tail. Though being a clutch, medium probably won’t make a noticeable difference.
I’m running 34t with 11-36 and that’s easy in capacity of a short mech Type 2, but running it for a bit on the full sus I was finding it struggling to shift into big cog, regardless of b screw position, and creaking when it shifts up there. Chain length was fine and accounted for suspension. It also ‘knocked’ when the suspension compressed, but Type 2s have known ‘knocking’ issues. Changed it for a medium mech and it’s fine. I put the short on the hard tail and that’s been fine on there.
34 on road, if you’re not going for it, it will do. Speed up and you can spin out. Personally I can’t go bigger for off road use though, not with the hills I do. If I was in Wales or somewhere like Lakes I might even consider a 32. That said there’s also the option of putting a 40t cog on the cassette. Not sure if that affects mech choice though.