Nikon 500mm f/4 VR or ?

Thank you everyone for your comments.

This would be my first plunge into quality and expensive glass. I've been using the Sigma 50-500 OS for the last year with mixed results. The price fit my budget at the time but I've grown frustrated with the soft images at 500mm.

I'm currently renting a Nikon 300 f/4 with 1.4 and 1.7 TC .  During the short period when the Sun was out, it worked well and the IQ improvement was immediately noticeable.  However, the remaining days have been overcast and the 300/1.4 combo struggles to focus and shutter speeds get longer making BIF shots more than challenging.

Actually, I have been studying the Canon 500 f/4 and 7D as alternatives - partly because of the increased pixel density and better video of the 7D. The older L model lens gets good reviews. A used lens and new 7d body can be had for similar money as a new Nikon lens. The newer Canon lens is also well reviewed and about a pound lighter, but also $2,000 more.  This would really strain my budget at this time.

If my main interests are wolves in Yellowstone, Bald Eagles, and Bluebirds from a blind near my nest boxes, is the 600mm worth the extra money vs. the 500mm ?  In other words, is that extra 100mm reach worth the price difference ?