My own view on the Merc is favorable. I own a 92 Crown Vic and the only thing that slows it down on low end is the 2::73 gears, it torques hard from low end and up. I have over 150k on the original engine (no oil use or leaks), the trans has been done, but that was due to lack of proper maintenance from the previous owner. I get over 23 mpg in my normal daily driving freeway/ street combined. The Merc has taller gears and the 3 valve engine. As far as pushrod engine developing more power than an OHC engine, I won't waste time on that comment.
The 300 is a great car, but a totally different market niche and much smaller inside. For a big boat, the Merc is great on the road, has a very tight turn radius, and is fun to take long drives in. It may be a big car, but has great speed for size, and great drivability comfort.
Just my opinion, but I do have a little experience with a similar car.
Mark