Is there any way you can swing an 06' and up X5? Two of my neighbors have X5's, and their parking spaces are side by side. But the e70 just looks so much better compared to an e53, when you see them side by side. Even my wife, who only pays attention to cars when I make her look at one, has commented how much better the e70 looks. They just look so much more muscular. This particular e70 has that Tobaco Brown leather interior, which is awesome, and the e53 has a black interior, so that makes this particular e70 look even better. She has decided that the e70 x5 is the vehicle she wants when we move to the 'burbs.
The Merc looks too generic for me. It does not have the personality of the BMW. I realize that is not a very thoughtful analysis, but it would be hard for me to get past that when it came down to buying one.
I have always heard good things about the Acura MDX. I know a few people who have had several of them, including one co-worker who trades his in every 2 years for a new one. But, he always gets the same color combo, which I think is a bit odd. I think it looks very generic as well. but it is hard to criticize the quality and durability of an Acura. I have never heard about any transmission issues, and did not see more complaints about it compared to the BMW on a quick Google search. It is worth looking into though, since an '04 would be out of warranty.
The Acura and the Merc both tend to blend in to the SUV crowd. The BMW's seem to stand out a bit more, particularly the e70. Have you done your homework on things like resale value, insurance, and cost of ownership estimates? I imagine the cost of ownership on the X5 is a bit high, compared to the MDX. Insurance is probably not much different between the three of them, but you never know. One may be less than the others. I would be interested to see how the resale values stack up among these three though.