guys,,, I am beginner in this hobby,, I just got my xray t3'11, and I am considering to get new brushless motor ..

what is the good one ?
but I saw in the motor there are a lot of differents turn,
it says,, 17.5, 21.5, 7.5, 3.5, what is that for ?
what is the different ?
and what is the good one for me ?

any suggestion I would be really appreciate it ,,