If you enjoy the large, ridin-around-in-your-living-room-feeling you get from the 300-SEL, keep it. Mine gets 20-22 mpg on the highway and it's there that these cars do best. As Sixto said, maint. on a 300-SEL is probably as simple as it gets unless you're driving something pre-1980.
If compactness and a zippier ride are your desire, check out the E320.