After a few years with 400 I don't miss my 300 at all except it had a better MFD. Bare 300 was seldom used and 420 not that much either. But my subjects are typically birds. Big game would be a different story though.What is it about this lens that you find desirable? Serious question. I know it is the "affordable" big white, but it seems to me that 300mm would be too short for most of what I want a big white for. I'd end up using it with a 2x converter almost all the time and would then be limited to "just" 600mm at f5.6. Not a "bad" option, but I wonder if the compromises of a converter would make it worth the investment. It seems like a 400 f4 DO (if an RF version is ever made) would be a more versatile lens at about the same price.
If you could buy only one "big white" would this be your choice?