I think that the 60 series had better header control options in the cab, the one piece hookup on the feederhouse and a different air distribution system on the cleaning shoe. I would think that in small grains the 9660 would be prefered to the 50 series. Don't have any idea in corn, how they would compare.
You could easily chip the motor on a 9660 to get the 20 hp.
Some of the later built 9750 combines had the 12.5 liter engine.
Power chips cost around $400-$600. Yes, fuel economy will be better. There are a number of different chips available for 9750's but there are 2 styles- '00 and '01 Tier 1, '02 and '03 Tier 2. These fuel systems are very different.
Don't ever buy a 9750 with the 12.5L unless you enjoy changing rotor belts, the "Canadian Special" has the same capacity as a 2004 9860 but the rotor drives are weaker. I have talked to guys that have them and they hang 1 or 2 rotor belt on the side of the combine cause they know they will need them. I would take a 9660 over a 9750 any day, the 50s are first generation are a little rough around the edges , the 60 series are more refined and reliable. Just chip the 9660 to make it equal to a 9760.