Tim,
Welcome to Eastman's Forums. I am sorry I did not respond earlier, but I have been out chasin' elk.
My philosophy is that a premium 50mm spotter like the Nikon ED50 is much lighter and performs at a level very near a good 65mm spotter. To really benefit from the objective size increase, it is my optinion you want to go to the 80mm/85mm spotters.
Now that is my opinion from owning some excellent 65mm spotters (Zeiss Diascope and Pentax 65 EDA). Others find the 65mm spotters the perfect solution for their needs. If I did go back to a 65mm spotter, it would be the Swaro 65HD, as it is not only high performing but also fairly light and compact, with its magnesium body.