roll cage

a good body will be better off than a roll cage IMO. a body will flex and absorb the impact like a shock absorber, whilst a roll cage transfers all of the force to the attachment points OR the energy is absorbed by bending the frame.
plus you suffer acceleration loss (most frames are around 10 to 15oz ) and an increased CoG (poor cornering).
good pro-line or jconcepts body is all you need....seriously.