Just got a pair of YS120's from a guy and one is supposed to be a 120NC and the other a 120SF. Does anyone know exactly what the difference between the two is? How do they perform in relation to the newer generation 120's? Thanks for any info you can provide!

The 120NC is slightly more powerful than the F-120.
The F-120 is the original production of the 120 series dating
back almost 23 years. The F-120 was last produced almost
14 years ago and preceded by the 120 AC. The 120 NC
followed. There are big differences in the carbs and
internally such as piston design,crankshaft design and
intake valve and exhaust valve diameters just to name
a few. The F-120(or 120SF) and 120NC Were excellent
performers on 10 lb airframes and could fly airframes
weighing as much as 16lbs. The newer engines,120SC
120FZ crank out as much as 6/10ths of a horse more.
Personally I think the 120SC was the most powerful
of all. It took advantage of a high compression ratio
to put the super charger at full affect.

Any idea what size prop an SF would best be used with?

14x10,14x12,15x8,15x10,16x6,16x8 are good choices.
If you want to hover try the 16x6.

Thanks for the info!