As an owner of a 2002 ML I must say there is no comparison; for sheer handling and driving experience, the X5 will win hands down especially with the sports handling package option. However the ML has the option of 7 seats, low range and more interior space.
As far as the engine is concerned, you can't go past the 3 litre 6 cylinder common rail direct injection diesel in the X5 - what a performer. It is a better (and cheaper) package than the petrol motors (not to mention fuel economy which is staggering) and with 480-500 Nm of torque it leaves my 2.7 litre common rail direct injection engine (400 Nm) for dead and the MB unit was state of the art 5 years ago.
Worth considering as well is the new Land Rover Discovery 3 which has just been released. It is a giant step forward with 7 adult seats but weighs in at 2.7 tonne. It has a single turbo version of the Peugeot / Jaguar 2.7 litre V6 common rail direct injection diesel motor producing 440 Nm of torque. Haven't driven it yet but it certainly looks the goods.
The most amazing thing is that none of these trucks will use more than 10 litres of diesel per 100 km. And that's with seriously spirited performance.
I'd say we were spoiled for choice at the moment which should keep the manufacturers honest.
Just out of interest, the MDX middle seat just has to be sat in to rule it out. A good first attempt but they have a long way to go.
Should you buy an ML, buy the upgraded (2002 or later) version as the earlier model had serious problems.