Need help regarding LRP Vector X-11 Motor

[Funkymojo]

Hi guys, i'm planning to get a LRP Sphere Competition 2007 Edition for my 4wd buggy. Which would be a better choice of motor? The vector X-11 4.5 or the vector X-11 5.5? My local track is medium size, i want to have good speed and controllable speed too...my mates told me a 5.5 is more than enough power in a 4wd buggy...any advice? Cheers