[suicideneil]

Depends what your goal is specifically.

A speed machine on lower voltage would need the 1.5d, but woud tend to run warmer and push the batts more.

A speed machine on higher voltage would use a lower kv motor, to give much better temps and runtime to boot.

Normal basher would be fine with the 1y on 4s, then stepup to 5 or 6s for speed runs and freaky power.

As for voltage- I would never say use 3s lipo for an MT. With anything other than the fastest 1515 sized motor it wuold be slower than old people humping, and if you then stuck 4/5/6s to the same motor... hot hot hot.