Man, tough question...
It is.
I finally decided on the Canon EF 1200mm f/5.6
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The main reason is because this lens is virtually unobtainable, and rumors are that they're phenominally expensive (rumors of $100K++).
As such, my thoughts are that I could eventually afford any other Canon lens in their inventory (if by no other means than to rent this one out occassionally), but I'd never otherwise have a chance to get to this one....
-hh