You would want the 1515. The 1512 would be to small for the Revo.

The difference b/t 4 and 6s would be the top speeds. On 4 s you will see a big difference in top speeds vs. stock.

Most people get the 1y, 1.5y or the 2.5d. I think it depends on the batteries you intend on using.

I am getting the MMM castle Nue combo whenever it is sent out. It is a 1y motor.

(I am new to the electrics, but am learning quick)

-Brett