OP,
As others have stated, if you're looking for a reliable and cheaper to repair SUV, then go with the MDX. If you're looking for a nicer, more luxurious, better performing/handling, and better looking car but are okay with lower reliability and higher cost, then go with the X5.

I was in the same quandary before we bought our X5. It was down to the MDX and the X5. After sitting and test driving both cars, there was no question which one we loved more. Don't get me wrong, the MDX is nice too. But, it drives like a typical SUV and feels only a tad more luxurious than a regular Honda.