Having mounted an OEM, I'd be leery of the aftermarkets I've seen. I don't recall whether e30cabrio, who removed an aftermarket to install a salvaged OEM, saw damage on his. I expect its one of those things where the aftermarkets are fine until you try and haul closer to the limits, which may be pretty rare given the use most put the E53s to. The most I have hauled to date is approx. 4800 lbs of trailer and hay on the OEM. I have pulled horse trailers for 10+ years, always behind a Ford superduty diesel, and don't recall ever seeing an BMW SAV pulling a trailer at horse events ....
Our 3.0s, even with a manual tranny, are tested [performance wise] hauling that much weight. That said, the OEM hitch appears to be a very hardy design.

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.