Go with OEM. That's what I did, towing a 5000lb travel trailer. It's rock solid, and tows like the trailer isn't even there. Yes, it's expensive to buy, but it's a one-time purchase that will last for the life of your rig. The few hundred dollars you could save by going aftermarket would quickly evaporate if the hitch somehow screwed up your X, or caused your trailer to get damaged.

My hitch installer was a local aftermarket specialty "truck toy" place. They do lots and LOTS of hitches, trailer brakes, and so forth. The guy was very impressed with the quality and stability of the OEM hitch, and said if he could so it, he's sell a hitch that good for all of his installs. I think he was pretty serious.

Dave

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2003 4.4i (The X5 is WAAAY more fun than my Jeep used to be... )