you will like the 13.5 better than the 3900...most likely, the 13.5 will be sensored Meaning the powerband will be alot smoother than the no sensored set ups.
13.5 is plenty fast on a track. i run a 13.5 vector x12 Lrp and a SXX v2 on my blitz ese and its plenty fast.
the 4 pole 3900 has a lot of torque compared to the 13.5