don’t just look at the rear mech, it is a common marketing activity to upgrade the rear mech, and usually to cut corners elsewhere.
If you have the choice of XT rear mech and Deore shifters and STX mech and shifters, your better getting STX for both parts. If the choice is Deore rear mech and XT shifters i would say go for it, but i doubt it ever happens.
Trek bikes are fine, worst case you get a decent warranty and back up, same with Specialized. But neither are very flashy or cutting edge.
To be honest there are very few bad bikes around at that price, it is more a case of unsuitable for what you might want to do. Do you want to play on jumps, or start XC Racing, or a bit of everything. Look at where your going to use the bike, big climbs and heavy bikes aren’t much fun, but a strong bike can take a bit more abuse on the way back down.
Consider decent forks if only because you can get the spares when they need servicing. (seal kits)
Ignore tyres, and ignore what tyre threads, just look around at what locals use, and learn by experimenting. How you ride, what you ride and where you ride all affect the way tyres work.