Not sure how much the 370 costs to maintain, but the GTR would fit the bill for most of that - not sure about 20k a year though. I have had mine a little over 3 years, but its a second/third car for me and I am happy to leave it with my mechanic for 2-4 weeks at a time. Its great on the motorway, lots of space, etc.
The issue would be Nissan again.... I refuse to use Nissan, unless I absolutely have to! They are also not the best at being speedy to correct an issue.... when my bellhousing became a problem, they want to take the car apart, check the part send notification to Nissan, wait for approval of warranty, order the part and then fit when they had time. All in, they said 6 weeks........ Sly did the work in less than 6 hours and ordered the parts 2 days before.
How many dealers are going to be any better than that? I seriously doubt that Merc would be. I tested the C63 AMG and loved the noise, didnt like much else (seating position was weird, handbrake is very weird and its just a standard car with a v8).
My missus had a Range Rover from new and it was back with them every 3-4 weeks and they were utterly useless!!! I think its more the service of the dealer than the car!
With that in mind, my recommendation is 2x GTR 1 to use, 1 to have serviced