Reman means brought back to new spec, under very controlled conditions. It may or may not be tested but most are. It isn't your transmission, it is an exchange. If it is from a reputable source it is better than a rebuild. I would only buy a ZF reman if I decided to go reman. That is what BMW sells, they don't reman them themselves. I would go to a dealer, then, if I wanted a reman. It is faster as well.

If I wanted a repair it would be because my core was in good shape and had a specific problem. I would do it myself, but if I paid for a repair it would be from a shop familiar with that specific ZF transmission.

If both suppliers are equally reputable, then the reman is a more complete repair. It may not be necessary, depending on the specific problem.

If you are dealing with non reputable suppliers, all bets are off.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue