I would be lusting for 600 but I refuse to pack that kind of bulk and as a result I don't get those distant shots that others do. However, I'm getting some of what others do not when I'm creeping through the grass and bush and sitting in the briar patch smelling the roses. It's always about compromises. This is an endless debate. +1 for my needs.
Also, cropping a 300mm to get a sharper 600mm? Nope.
Also, back to what I said about environmental factors. I shot a lighthouse this morning from about 1 mile away with my 300+2x combo and even though it was 35°F, the humidity (somewhere around 50-70%) mucked up the shot with a lovely mirage pattern (waves), so depending on where you live it can be a small or a huge factor. See below for a 100% crop, straight out of the camera.
Well, if your prime use is distant architecture, then you´d run into these problems in most places on the planet
If I disregard a few moon shots, I have hardly ever shot anything more than 100-150m away. The majority of bird shots are probably on average 50m away. I believe you would be able to do that in most places, regardless of heat and/or humidity.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1