Karasa, I am also trying to decide between the Canon 500mm F4 and 600mm F4, but I'm considering version II.

For me it comes down to ability to travel with the lens. I gather the 500 will fit inside some medium to large backpacks, these packs will fit inside most overhead bins in commercial airplanes. I'm pretty sure the 600 will not fit in any backpack and will have to be carried in a separate bag/case.

Personally, if I only shot locally, I think I would much prefer the 600 as the added reach really helps. I have not used a Canon 500 F4 yet so I really can't comment on the quality.