I am the same height at 5’10 1/2” and 189lbs. I too fit smack in the middle. I have a Medium Specialized Epic, Medium Cannondale F-29, and Team Scalpel in Large.
If you want to ride more XC I would get a Large as it is more stable, peddles well, and is straight-line compliant. If you ride Single-track or more All-mountain, and want to flick it around and cut the turns I would go with the medium. More down hill with big drops and lots of obstacles, go with Large.
I hope that helps.