At one time I owned both. For birding you want as much reach as possible.... period. The 600 is a lot more difficult to travel around with and everything is a tradeoff. What i do is use a 400DO and 600 when i shoot birds. The 400 DO can be hand held for quick action shots. The 600 remains on the tripod at all times. I did sell my 500 I am 5'6" 150lbs and I agree with MR's review that the bulkiness of the 600 makes it much harder to carry and set up but with the right bags etc, I still do it. I am 49 yo and in 5 or 10 years perhaps i will downscale..
I assume you've seen this review.