I can't speak about reliability yet - I have had my X5 only since Friday. But based upon past experience w/other BMWs, I believe the MDX would be more reliable. Having said that, I do not anticipate that the X5 will be unreliable - just that an Acura (honda) product should be more reliable. In the 3 years I had my 540, I had a couple of major problems with it - but the enjoyment and fun factor were very high. OK - MDX - wins in overall room, reliability, seating flexibility (seats 7 in a pinch). X5 (I got the 4.4) - wins in every performance parameter, looks better (significantly), much more desirable. It wins in my book. By the way, I did not even look at the ML - had a bad experience with an E420 before I got my 540 - won't go back - once bitten, twice shy!