I had the 500f4IS for 3 years and felt it was a good length for wildlife and birds, even for a female. And if you shoot larger animals other than birds, the 600 may be too close much of the time. The added weight is a real issue, unless you are planning on a full time tripod. BTW I bought the new 500 II this month and the weight difference is substancial, enabling me to hike further and hand hold longer. A dream lens.
I added a 2x just for reference and am amazed of the clarity at 1000mm!! My 1.4x could also be used full time, no problems with focusing on BIFs. Clarity excellent with the new lens.