Why are you interested only in 31" hookahs
? I think 28-35 is the best all round. Including performace, portablity and looks. At those heights any Mya , KM , MZ or quality syrian should be perfect.
If you want a multihoser (which i wouldnt recommend) Mya is the only way. If you want top notch performance portability and looks something like a Mya Gyro is perfect.
If you want a heavy duty hookah that is most likely to last you a lifetime at a slightly better price a KM or MZ is the way to go.