For one, the 5 port is cheaper but you won't see the difference unless your track is reasonably "big" or has a long stretch of straight. I have both and I usually use the 3 for smaller/street tracks, and use my 5 for the larger and straighter tracks.
Be sure to spend time breaking in the engines. The OS take some effort but once done, they are great.
Serpent 960-08 + Max XL9
XRay NT1 + Max XXL3