the 55 and 500 should have not too much difference in fuel or economy. The AMG wil be faster, as its 60bhp up and its 400cc larger engine. (5.4l) On a long run the consumption will be little worse, maybe even better than the 350 car as its working much less hard. Both the 500 and 55 have the same transmission, similar weight & V8 engines so there may be only 2mpg in it
This was true with my 4.4i BMW X5 vs 3.0i X5. it was better on fuel on mways and long straight roads. The 4.4i was truely awful around town and could be single digits mpg wise. the litre car would remain in the teens in stop start. Twisty stuff with on off application of the gas pedal would also see the 4.4i cars fuel consumption plummet in comparison to the 3.0i's.
If you do town driving and short country road runs there is a real case and point for the 350 car. If its mainly mway and trucnk road driving, and fuel isn't an issue there is no reason not to get the 500 or AMG.
The AMG will be very costly to service, even relative to the 500,
