You can run either voltage, just expect lower torque and slower speeds at 4.8v.
For a warbird these will be great, but I wouldn't use them on an aerobat.
Would probably go with the HD 9150MG (btw, Hobbycity.com has these for $33.80.... AND TBM will match that !
Supposed to be GREAT !

http://www.hobbycity.com/hobbycity/s...idProduct=6334
Jim