Hi...I just got a second hand,unused EDIIIA..this is the last version of the ED60 nikon,and it is waterproof..I am mentioning this because nikon has upgraded a couple of versions after the original ED60,and not until the EDIII that they waterproof the scope(O-ring ,Nitrogen)...If You can find a used,in good condition EDIII,for the same price than the ED50,I think is much more scope in all regards..
Still small and not too heavy in comparison , even considering the super-ultra-light weight of the ED50..the EDIII weights 1095g. vs 450 or so for the ED50..
The EDIII is super nicely built,,robust,light,and amazingly nice ergonomics on the tripod( I have no issue with it being back-heavy)..
The center focus ring is much nicer to use than focus knobs that I have used,for many reasons. Is better damped than most,and offers a perfect balance between resistence and fluidity,plus a very good speed ratio,that is both easy to use in a fast or fine way.
It is also easy to find ,left or right,independent of the eyepiece orientation of Your choice...And the orientation of the eyepiece can be changed with the rotatory collar,also a very tripod-friendly feature..In that regard ,ergonomy and durability the difference is clear and the advantage goes to the EDIII,,,Optics are also better,..as nice as the ED50 is in that regard,the push in aperture is very significative,and with the great correction the EDIII has, It really delivers ..If You need 30X or more ,there is no comparison.All I can say at this point is that there is NOTHING I dislike about this scope(the EDIII)
The ED50 has many charms in itself, no doubt,but there are issues....It is so tiny ,Is great optically and Is not too expensive..then is the issue of being able to get away with a compact tripod,also a plus...Durability issues have been raised,and few users in this forum only,had reported the scope breaking..This point made,I personally would be more worried about buying a used ED50 than to buy a used EDIII..
So finally Your question was not about what scope is better ,or about personal preferences,...You were concerned about if It was better to buy new,with a full warranty or used with ...What warranty?....
You are in the USA and Nikon has a No-Fault warranty in their products that might or might not work with second hand optics...My experience is that It does, but even if not free,or No-fault ,the products have some implicit warranty and Nikon has good customer service ,so You will be able to find a solution in the event Your second hand scope needs service..