Amp draw/efficiency question

[What's_nitro?]

Having to use a smaller pinion with the faster motor would (I think) reduce current draw during startup since the motor can spool-up easier. However, the faster motor has a higher no-load amp draw. If that translates into the loaded amp draw then it may cancel out, or even negate, the benefit at start-up.

[nuz69]

This thread illustrate the "sweet spot" issue of a brushless motor.
Low Kv and low voltage isn't good, high Kv and high voltage isn't good either. You are comparing Low Kv at low voltage and mid Kv and at voltage. The low Kv motor has smaller thickness of its widings wires, that's why it does'nt like high amp draw and need more voltage at same power, but that's why low Kv keep the ESC cool and happy.
==> I would say that geared for the same speed and at the same voltage, the higher Kv would be best.
It's also for thoses reason that we speak about "sweet spot" for brushless motor and why a certain minimum and maximum RPM is advised. For the 15 neu series, the "sweet spot" (according to most users) would be between 35 and 40K, so IMO the 1,5D would be better and run cooler at the same power.
NuZ