2014 Mercedes-Benz M-Class Pricing and Specs

If it is a diesel I would have the particulate filter checked, it might require regenerating.

As you say Mercedes generally cost more to service and repair, particularly if you have a dealer do the work.

They're not as expensive if you use an independent Mercedes specialist, and there are plenty around who can do that for you. They not only charge less for their labour, they usually have access to cheaper parts. Most of those parts are full factory parts, just sourced from overseas, instead of locally, so you don't miss out on quality or reliability by using them.

Jeep owners generally swear by the cars, but they have had issues over the years that should make you wary about signing on to the brand.

There's not much to split them, perhaps I would prefer the 'Benz.

You're correct in thinking the Mercedes would cost more to service and repair, particularly if you use a dealer. Going to an independent service mechanic with experience of Mercedes-Benz cars would save you plenty. The Jeep would be the cheaper of the two in terms of servicing, and of the two you are looking at that's the one I would opt for.