If the carboy is 6.5 i'd say use that for primary. Easier to clean all of the trub when your done with it, u can see the fermentation as its going on, and if you get a big enough tube u can put it right into the top, instead of using a bung/stopper/cap for the blowoff.
Either way, def invest in one of these...
http://www.amazon.com/dp/B0064OEKCK/?tag=skimlinks_replacement-20
Glass is slippery, no matter what your doing with it. Save ur feet and floor (and carboy) from an accident, these aren't expensive at all and will help with the transport.