op-amp vs direct control
A few problems with the op amp circuit:


Depending on your motor, its highly likely that the op amp cannot source enough current to drive it.
The op amp is running open loop. The output will almost always be saturated at the positive rail because the inverting pin is pulled to common. You may have a few tenths of a millivolt of control, depending on the open loop gain of the op amp.
\$100\Omega\$ is generally a low resistor value to be using with op amps. Use something in the \$k\Omega\$ range.


The potentiometer solution isn't very good either. Putting a low impedance load on the wiper of a pot like that will result in nonlinear taper. Only high impedances should be driven from the wiper of a pot.