I'm thinking about selling my 500/4 IS and replacing it with a newer model. The obvious candidate is the lighter, slightly shorter 500/4 IS II - and lighter weight would be very, very welcome, as would the shorter MFD. On the other hand, the new(ish) 600/4 IS II is near enough as makes no difference to the same weight as the previous Mark 1 500/4 IS I've owned and loved for years. Same weight, extra 500mm! That's tempting too. The extra length (about 80mm) does concern me though, particularly for use in-car. The 600/4 Mark II is around the same unweildy length as a 500/4 Mark I & teleconverter, if I have the numbers straight, which is awkward and clumsy but bearable. Not sure I'd want to use the 600 with a TC in a car though. (And no, I'm not going to buy a new car as well!)
I'd be grateful for any thoughts and experiences anyone has on this topic, or more broadly about the big lenses. (I'm not actively considering a 300/2..8, a 400/4 DO II, a 200-400/4, or a 400/2.8, but any remarks on those would be of interest also.)
(If it matters, I'll be retaining the 1D IV as my primary body, and these days mostly (but not exclusively) shoot bare lens - i.e., I seldom use the converters, though I frequently use a close-up ring for small birds. I shoot both hand-held, with a tripod, and from the car about equally often. For no reason I can explain, I don't use monopods. Just never got the habit.)
Edit: I'm not going to worry about the price difference. In reality, they cost the same - one grandmother or a couple of kidneys.