Im 5' 8" which size toptube is right for me? Right now i ride a 20.75 bmx bike, but im gettin a mountain soon?

right now i ride a 20.75" toptube bmx bike, but im gonna be gettin a blk mrkt mountain bike, and im 5'8" which size toptube is better for me? the wheels are 26" and which is better for me? 21.5" or 22"