3905 tranny or Gorilla maxx single speed

OK i ask this question because i don't know if i'll buy a brand new 3905 or if i'll buy the e-maxx of one of my friend. His e-maxx has alot of hopups, gorrilla maxx single speed, 3905 diff on both end, Traxxas steel CVD, aluminium chassis brace and lot more, and i've it for a realy good price.

I want to drop in my Neu 2.5d, with my MGM 16018 and a pair of 6s wired in paralel.

OK another question, is i'am beter to take 2x 3s in serie or 2x 6s in parallel