mercedes is german, maserati is italian.
mercedes has a large product range, maserati does not
both have a very long history even in racing
both will be expensive to buy and own however on average, due to the greater range of mercedes product, mercedes is cheaper.
personally i think maseratis after 2006 look better than mercedes but prior to 2006, mercedes looked better.
mercedes has an F1 team, maserati does not
mercedes has commercial vehicles such as trucks and cargo vans, maserati does not
mercedes dealerships are everywhere, maserati dealerships are not.
if I had the money and was specifically restricted to either a maserati or mercedes i would buy a maserati.