How would you build an Ultimate Towing Machine (E70)?

[RickM5X3]

Trying to figure out the best way to go, as I'm going to make the jump to a fresher (but used) X5 for the upcoming season. Travel trailer is 22 feet and 4,500 pounds fully freighted. Usually have 4 bikes on the roof of the X5 as well. What would you pick for the ultimate towing machine?

1. X5M - seems like an obvious choice, as the power, braking, and handling are more than sufficent. Comes with rear air suspension and adaptive drive standard. Do I get a much better cooling system than the other choices? Hitches are available and there is a minimum of fuss on the install, right? Only downsides are lack of 3rd row, which would be nice to have, and low clearance might be an issue on dirt roads.

2. X5 50i M-Sport - seems like a good second runner up, but I think I've read that a hitch is not so easy on this one because of the bumper design. True? Might 8 speeds be better for me than the 6 speed X5M? Air suspension is standard here too. Would do a dinan chip tune.

3. X5 50i with Sport activity package, adaptive drive, and 3rd row (in order to get rear air suspension). I like this idea b/c I will avoid the larger fender flares of the X5M and M-sport and the rock chipping issue. This will be a rare one to find on the used market, but if I do, will the suspension system as I've spec'd it get me close to the capabilities of the M-Sport? Would also do a Dinan chip tune for this one.

The 35d was omitted as I need something that is a fairly decent substitute for my old M5 when not hooked to a trailer.

Any thoughts or considerations that I've missed would be helpful!