If a bike is a bit too large you can put on a shorter stem and move the seat up an inch. Stretching out and dropping your torso,,low is fast on single track...
Go too far with this and your back, neck and wrists will hurt.
This mostly depends on your core strength,
Fat and soft In your middle = Pain and Six pack abs means No Pain, Simple
If a bike frame is too small reverse the above BUT you may feel cramped.
Top heavy, feel like your going over the bars, Limited movement on/around/over the bike. Once you turn those bars the longer stem will mean nothing.
Longer stems slow steering, tiller effect, I don't like that.
Shorter is faster and feels more precise IMHO, others may differ, call It ,'Twitchy'
Twitchy is bad. I ride a 60mm stem and It's not twitchy. More Important to me was riding a 'Slack' bike, My 66.7 degree head tube angle gives me much stability when I need it the most like Going down steep stuff, Railing berms.
Any time my speed comes up It just feels more stable, makes me want to go faster.
I bet this was no help..
If I was on the border I'd go Up In size, more aggressive/skilled riders who ride wilder trails may choose smaller IDK. I am dead center of the mark for Large frames and cannot ride a medium.
For the money your going to spend I'd want the Rosco, Second reason, Treks come In
SIX different sizes (I Ride a Scott Scale 720 Plus), Much like the Rosco
First and foremost I'd want the bike brand my favorite local bike shop sells.
I try to always support my LBS, I even will pay a few dollars more to buy from them.
What ever you buy
GET A DROPPER SEAT POST ! I use it more than my gears. I wont ride a bike without one and consider It a safety device just like my helmet