An S350D is pretty quick they will crack 120 in good tune and get you into jail. Fast no they are the slowest W140 importated. An S320 will walk away from one and comparing one to an S600 wouldn't be fair.
E300D's are pretty quick and get good fuel mileage. If you want good mileage keep it stock because to get more hp more fuel must be burned.
Both are good cars but like any W140 make sure the evap has been done.