If you're running stock classes the higher capacity will also give you more punch.
6000mAh * 60C = 360A continuous discharge
5000mAh * 60C = 300A continuous discharge
Basically the above boils down to the higher capacity cells having less voltage drop when you apply a load (ie. accelerate)
Having said that, if you are running on slippy tracks it won't realy make much difference, the lower capacity cells might even be easier to handle...
As for brands out there they are all good although I am a bit partial to the Vampire Racing ones
