RE: 3x9x40 vs. 3x9x50
I have scopes with 40, 44, 50, and 56mm objectives. The 56mm is too large, period. The 50mm scopes that I have are target models and would not really be suited for hunting applications anyway. I have 5 Zeiss Conquest 4.5-14x44 scopes and love them. I also have 4 Leupold VariX II 3-9x40 scopes that are on my guns that I carry the most. They don't collect as much light as the Conquest 44mm, but I have had them all for several years and have had no problems killing deer or hogs using them.
So to answer the question, I would not spend the extra money for a 50mm.
As a side note, I have never owned a Nikon, but I would go with it over a Burris. I have owned several Burris scopes and have hated them all. For me, if I didn't keep my eye in the exact middle of the lens, it would "black out" and usually at the worst most inopportune times.