I don't understand motors near like Justin. But he is right, the slower motor will pull exactly the same at 0 rpm, same torque.
However,, and this is where I get my ass jumped here, as you go from 0 to 10 mph, I think there is a big difference. And its not what you might think. As a motor gets closer to its max rpm, back EMF increases, casuing the motor to pull less hard. This happens sooner, or at least at slower rpm, with the slower motor. So some will claim the fast motor pulls harder. And indeed, you will see it pull more watts longer with a wattmeter. So the fast motor pulls more torque longer? Yes, sort of. Watts does not measure torque, and the fast motor will make more heat till it gets up to speed. So what is really happening?
Some will, by seat of pants feel, swear the slow motor pulls harder. It does not at 0 rpm. but at say 80 rpm, it may waste less wattage at that moment, and actually feel like it pulls harder. Its not more torque, but if given equal watts, the slow motor does have less wasted watts at slower rpms. So this is why some say the slow motor is the high torque model. But,, because of back emf being different, its kind of impossible for the two motors to actually pull the same watts at a given rpm. Very hard to make a true statement at any rpm other than 0 rpm then. ( they should also pull equal, at equal rpm relative to the max though) The bottom line is basically this, if you will be riding at low speed enough of your ride, then a low rpm motor will waste less of your batteries watt hours than a fast rpm motor. But only at low speed and only if the load is high. If the load is low, like flat ground, the waste is less. And riding slower, of course tends to be more efficient simply because wind drag decreases.
And to confuse the issue even more, if you have a big motor, it accelerates so fast any differences like this become meaningless. The big motor simply blows past that inefficient rpm so quick its not costing you any heat, or lost watts. If your motor wind rpm is high, then more power solves a lot of the efficiency issues. Except for drag from faster riding of course.
Too many variables, my brain is about to explode now.
Why you want the slower motor is so you have a slower top speed, for whatever reason. For example, adult trikes need to stay below 15 mph, or you can't really steer them anymore. 8 turn, 9 turn, etc are slow in that type motor IMO.
Or, if you will be overloaded relative to your power, such as a 700w setup on a 400 pound bike with rider and or trailer. Then the slow motor will overheat slower, climb a steep hill longer without overheating, and work better than a 6 turn motor in a big wheel. The solution to all problems is a smaller wheel btw. When I wanted a lower power bike still able to tow a trailer up a serious mountain pass, I built this.
This bike has the 9 continent type 500w rated rear motor, in the 9 turn version. Top speed 18 mph, but able to climb a three mile long 6% average grade at 400 pounds weight. Some of the benefit was the wind, but much more from the 20" wheel. At the top, the motor was warm, but not even close to hot. The weather, 105 F. No, not more torque at all, but able to tow a lot more weight up that mountain than the same motor, in 7 turn, in 26" wheel. That setup does make it to the top without melting the motor, but the motor is right at its limit at the top. One more mile would cook the motor with the 26" wheel.
This is why I say keep it moving 15 mph. With the 7 turn motor in 26" wheel you can climb more or less infinitely at that rpm, but less, as in the case with my test, (13 mph up the hill with 26" 7 turn motor and 400 pound weight) and you will hit a limit in length of climb. Rider only, and 300 pound total weight or less, no problem with the hill. Just a problem with only 1000w into a single hub motor, and 400 pounds.
With your mid drive and the 7 turn motor, you should easily be able to feed your bike 1500w continuous, and keep it up to 15 mph. With more power, you should not have any issues with the dd motor overheating.
Additionally, with the mid drive you can choose to climb only on the mid drive. Go slow as you want in the lowest gear, and just give the hub motor enough power to eliminate the drag, about 50w. You will need a watt meter on just the front motor of course, to know what is going on up there.