Seeing that you said you ride trails, I would go with the +2 arms. Anything +3 is going to be very tight in the trails, +2 is much more managable.
A wider axle is not an absolute must, but adds stability and better handling that's for sure. If you want to do it right, get a wider axle with some adjustability.
00' 400EX - GT Thunder 450 stroker, custom JE 12.5:1 piston,
+7mm stroked crank, HD stroker rod, HRC cam, GTT full race porting, +1mm intake valves, FCR 39 carb, CRF timing chain, Denso IXG27 iridium plug, full Hinson 8 plate clutch, Desert Toyz Supercooler w/ SPAL fan, UM airbox w/ UNI, CT Hi-Output exhaust, Roll LOBO II F/R, LT PEP 8 click ZPS piggyback fronts, PEP ZPS rear, YFZ brakes/450R master, CCP stabilizer, Hiper Tech beadlocks, Durablue X-33, TAG, Quadtech, ASV, AC, IMS, RAD, Works Connection, Galfer..
05' Kawasaki KLX 110 - Fast 50's springs F/R, HRC cradle, Rox risers, FLY fatbars, NGK CR6HIX iridium plug, Twin Air filter