I am going to try to complete a super randonee series this year and wanted to see what everyone's opionion is on which bike I should use.
Why would you assume a single bicycle is preferable for all brevet and all conditions? Do you carry the same gear for a 200k as a 600k?
I worry about this one on the longer (400 and 600) rides as it still is a little more aggressive than the other two.
If you use bikes that fit differently, and "worry" about longer rides, then why not change their fit to suit your purpose?
Instead of thinking about your bicycles, why don't you think about yourself? Make a list of the various traits of each bicycle that may or may not affect your brevet riding experience. After listing all the factors that you can think - start to assign a priority to each bicycle's factors as to their importance.
You should be able to come up with the "best factors" bicycle. Of course, this whole thread is silly, because the brevet conditions constantly change. (as well as route and distance)
My own experience suggests that "bike selection" strategies work out when you know what kind of weather you will be riding in and have a good feel for how you want to "ride" the particular brevet. I'm not sure you can do that very well from a keyboard -weeks in advance.
In any case, use the bike that "fits" best.