Casting no aspersions on either company, you should make sure of a couple things when purchasing. Be sure that no chroming has been done I.E. Gates and or Rails. That if you buy, say an Arri BL 4 or SR III, it has an Arri BL4 or SR III motor and not a BL3 or SR II motor. The mirror is pristine and the mags have recent clutch overhauls. The electronics are free, totally, of any corosion. Also the serial # is checked through PERA the rental industry hotline for stolen or "missing" equipment. If a cam. has traded hands more than once the seller may not know its "true" history. When getting lenses I highly recomend they be shipped by the seller to a lens tech such as FOCUS OPTICS for a complete checkout. Especially, a zoom lens grease packing for a good focus of the gear system is unfortunatly not an uncomon practice in zooms or primes. There are more pitfalls to consider than these but these are some of the main ones.
Best and Good Luck,
GWPB