I'm currently debating the supertele choice. With the costs involved, it'll likely be next spring before I move forward...

My primary use is birds/wildlife, which I currently shoot with a 7D and 100-400mm. I often need to crop the resulting images a bit.

I had originally been planning on the 500/4 IS (MkI), as the longest reasonably handholdable lens. But I decided to use those funds for a 1D X, then the original 500/4 IS was discontinued anyway.

I then planned on the 500/4 II. I'd really like to use the 1D X rather than the 7D, for the lower ISO noise, better AF, and faster frame rate. But, 500mm on FF means giving back 140mm vs. 400mm on APS-C. OTOH, the 600/4 II is essentially the same weight as the original 500/4, so about as handholdable. Giving back 40mm isn't that big a deal, and with the 1.4x III, I'd gain 200mm relative to 400mm on 1.6x, and have great optical results. So now, I'm leaning toward the 600/4 II.

Of course, I've also considered replacing the 7D with a 1DIV, for the 1.3x and f/8 AF. But, the 100-400 + 1.4x won't give good enough optical results, meaning I'm back to the 500/4 II or 600/4 II (although the 300/4 IS + 2x is a possibility, I'd prefer to stay away from the 2x with f/4 and slower lenses). I'd also like to be able to use all the AF points, not just the center one.

So...it comes down to three main choices for me, and the ~$1000 cost difference isn't too important (since it's <10% of the total):

1) use 1D X, buy 600/4 IS II and 1.4x III (840mm f/5.6)

2) use 1D X, buy 400/2.8 IS II, 1.4x III, and 2x III (800mm f/5.6)

3) buy 1DIV (used/refurb), 500/4 IS II and 1.4x III (910mm f/5.6 FF-equiv for exposure, f/7.1 for DoF)

Decisions, decisions...