When it comes to a 50-60k engine versus a 30k engine, chances are higher the former has more problems (clutch needing redone, etc.). I think I read on here the nighthawk valves don't need adjustments but I don't know if that is true. I wouldn't be afraid of either one, honestly.
The telling differences for me would be the condition of the inside of the gas tank, rust on the bike, etc. That will tell you if the bike was neglected.