I read some of the posts for soldering direct vs plugs and thought of another reason to use plugs: reversing the motor direction. If you decide to mount the motor in another vehicle, then reversing the motor direction is much easier if you can unplug them.
As to soldering the plugs, I use a hot iron (100W). I tin the wire leads and the inside of the plug hole, then insert the wire in and heat the connector while feeding solder into the hole until filled (shouldn't be too much). Works well and looks good for me.
I've also soldered direct and found a trick that works quite well. I get a crimp-type yellow
butt connector, remove the plastic shell, and then drill a 1/16" hole on one side of each end. I then stick both wires in each end, but don't crimp. 12GA wires are actually snug in the hole. Then I heat the connector and feed solder into the hole i drilled into the side. When cool, I slide heatshrink tubing over the connection. Because the thickness of the connector walls is about the same thickness as the wire jacket, the heatshrink almost looks like there is no connection with no solder bulges.