Well for me the 500mm most likely is enough for my birding usage since the ones i most love to take pictures of allow me to get close enough =) Also the backback is no problem for me since if i am gonna get the fstops tilopa with xl insides. The price difference on used 500 and 600 is quite big still here in finland used 500 is around 2.2-2.5k euros when 600 is around 3.4-3.6k euros.

I would naturally love to have the newer version but that is soo far from my budget still