help me choose a motor

[bfdbh]

I want to use a rear wheel sprocket adapter and have a left hand side chain for the motor.

Do I need a motor with gearing?, how does the balance between rpm and load work?

thinking of this motor: 2500rpm http://www.xtreme-toys.co.uk/mini-dirt-bike-36v-500w-motor-with-11-tooth-sprocket-my1018e.html

or this one with 7.18:1 gearing, rated speed 3k rpm, actual speed 390 rpm:https://www.dhgate.com/product/my1018-450w-24v-36v-gear-brush-motor-with/389031417.html