Is it going to be –
An all day xc bike with 120mm forks
or
Aggro trail bike with 140mm forks and a more meaty build
Im 5ft 10, 31″ inside leg, the lack of standover on the medium really annoys when used in an aggro fashion. The tt on a medium with a 70mm stem feels too long for me too, but then im not hung up on a mtb fitting like a road bike in the saddle.
Id say xc, saddle up all the time, medium will be fine for you no doubt. It’s just that if you want to thrash it, that muddies things some more, id be inclined to say small based on my preference and what i think will work for that purpose, what you like will obviously vary.
Demo a small if you’re thinking a little more aggro just to clarify things, if it’s xc, you won’t go wrong with the medium.