I am going to Bosque (del Apache) in about 6 weeks. I'm a Canon shooter, 5D and 20D. This will be my first time there but I've researched this to the extent possible. I am taking...
70-200 IS - probably will my most used, from what I can tell and what I want to achieve (environmental portrait of several birds, flyaways)
300 f/4 IS - I am most skilled hand-tracking airborne subjects with this. I'll probably couple it with a 1.4TC at times.
400 f/5.6 - because Art Morris insists this is "the lens" in all his writings. It is small enough too include, but I am leery because of no IS.
500 f/4 IS - If anything stands still long enough for my feeble skills with this monster, I will give it a go with my Wimberley head. Individual bird portraits are not always the primary goal for folks attending Bosque, but this is a once-in-a-lifetime opportunity for me, so, I need to bring it and try.
Based on all of this gear, I have decided to drive rather than fly. I have two other mates in the SUV to share the expenses and the driving (2.5 days on the road from Cleveland). If I didn't drive, I would cut back. Others in our party that are flying are finding air travel restrictions a real pain in the arse.
Most important item: Gitzo 1325 with RRS 55 and full Wimberley head.
To answer your specific question: the 200-400 is WAY more useful than the 600, according to normal usage patterns.