Motors aren’t limited in voltage like the chinese specs say and the ”specs” are often mixed with numbers that don’t go together. The 6374 motors will be able to take 12s if you want to use them. Outrunner motors will not explode like inrunners can so the limiting rpm will be when eddy current losses get too high and cause heating and loss of efficiency. The 80100 motors i’ve used could take up to 7000-8000rpm before eddy heating started to get noticeable.
Energy in equals energy out plus losses so these motors will both take roughly the same power to do the same task. The difference is the max torque and power potential which will be higher for the 7070 motor with its larger airgap area. It will also have a bit higher efficiency at the higher torque levels
If you plan to use it for an efoil then i would choose the 7070 to have a bit more power available to get up on foil. There are some people here that could get up on foil with the 6374 but i think they might have been lightweights😀
Then for the different kV figures, torque potential of motors are not set by kV but by the saturation and heating of the stator steel and your controller and battery max current so a 50kV motor fed with 100A phase current will create as much torque as a 100kV motor fed with 200A phase current (as long as we’re talking same motor only different kV)
it might seem like the higher kV motor is drawing more current to do the same task and would be less efficient but eventually there is only half as high voltage needed for the same speed and this evens out to a few percent in efficiency difference when motors are compared. Normally it’s best to choose the lowest kV you can reach your rpm goal with but you can get away with a lot if the system just is well balanced in motor-esc-battery capability.