thanks for all the comments.

I am still not getting a clear answer to the basic question, which is this: how should I evaluate the following two choices?
-3000kv motor with a 15 tooth pinion
-5000kv motor with a 9 tooth pinion
(both motors running on 2s)

Both setups will get to the same top speed (at least on paper - and that's good enough for our discussion here).

The main thing I want to optimize for is heat - which set-up will run cooler? The one in which the motor spends most of its time at 45k RPM, or at 25k rpm?