Self Propelled Swather Horsepower?

Looking at used, late model, swathers (Massey Ferguson). See many that are set up similar, with 16' disc headers, but are different horsepower (MF 9760 vs. 9770 or 9860 vs. 9870).

What are pros and cons of each? Hills vs. flat fields an important factor? Speed while cutting?

Would I be dissatisfied with a lower horsepower machine? Heavy first cutting, steep hills, rough ground (slow ground speed required).

Thanks for any advice/wisdom.

Self Propelled Swather Horsepower?