Mariner 40hp Twin Carb 1985 VS Mariner 40hp single carb 1998

i have just picked up these motors, tiller steer short shaft both ex rnli. (lifeboats)

Just wondering which outboard will be best to keep, the old one is a yamaha based motor and been well looked after and serviced, the 1998 has stood for a while and the tilt has seized so looking to get that undone asap. both run and engage gears.
the 1985 has been repsrayed so looks tidying from a distance and the 98 has a very scratched up hood, so need to replace it really, but the rest is ok.

Which one will be more fuel efficient and will there be a noticable difference between them (assuming they are both fully working to potential, compression test to follow)

Any experiences of either motor appreciated.
i think the 1985 40hp mariner is about 65kg (143lbs) and the 98 is heavier at 75 kg, and as i willbe moving it and mounting it on the boat on my own i am taken toward the lighter engine although the 98 has gas tilt assist!!! nice.

thanks

PS. cant find the serial no on the 1998 motor (its the later type so guessin circa 1998 and previous owner said it was?!)
where should it be located and can i find out the year from some of the numbers on the engine block??

Much as I love the early Yamaha based twins, I'd go the newer engine every time.