you are thinking about this the right way. the problem is that the max rpm doesnt indicate where the torque is. you question is about temperature though. assuming both motors have peak power at about 50-60% the the motor will run cooler at 70-80%. above 85% the motor get inefficient very quickly but if you can add timing to weaken the field at 90% would be the best because you will spend less time in the very low efficiency zones during acceleration.

the only drawback is you cannot gear the car in a way that it goes over this 90% whereas you can gear the other one to do so.

thats why having a motor for low end and another for high end rpm covers all the bases.

if it seems like im talking in circles Im not trying to. Im trying to give a little info to a question that isnt complete. its not your fault your head is in the right place. your thinking about this in a different way than most people do and if you get fluent in it you will know how to pick a motor properly.

this is the data or questions that will get you where you want to go and not all of it is published so you may have to generate some.

where is peak power of the motor (normally 65% of rpm) but I dont know all motors
where is peak efficiency of the motor (85 ish %)
when you calculate the average rpm of a race take that number and put the point on the curve for the 2 motors at the above mentioned curves.
in order to do that you will need to look at the highes fdr and lowest fdr possible in the chassis.

it will be obvious to you once you do that because you will pick the one that gives you the most room for gearing and small adjustments between efficiency and power.