ORIGINAL: DriveWRX

I'm using a Just Go Fly 400DH BL motor w/8T and a CC Phoenix 25. Works great!

One other difference between the ESky 3800kv and the 3900kv is size. The 3900 is a drop in. The 3800 is bigger so shaving of the frame is needed to get a proper fit. However, the 3800 is more powerful.

The JGF 400DH is a drop in and it's rated at 4200kv. It does cost more.

Any will work, so it's your choice.
400DH: drop in, powerful, costly
3800: needs mods to fit, good power, cheap (price, not quality)
3900: drop in, least powerful of the 3, cheap (price, not quality)