I understand doing research, goodness sake I myself analyze decisions way beyond useful consideration. When considering getting a long prime lens, I read/studied all of the objective (aka test data) material I could find on the optical qualities of the current 400, 500, and 600mm primes. But once a decision is made on the focal length, what's left to consider? Used or new, older AF or AF-S, VR or not, etc. The optical performance is what it is. One model is ultra sharp, oh but the other one is ultra, ultra sharp. Though, and I'm loathe to bring this up, at one time I did consider switching to the brand that shall not be mentioned because the lens I was considering was of more value than the rest of my kit combined. And at the time a pro body and the focal length prime I wanted could be had for what the Nikon lens alone cost(at the time). Barely escaped a lifetime of regret on that one....