I find using the reamer quicker and much easier to do. I played around with the K&M, but the shell holder difference caused me problems with wobble. I also played around with the Forster neck turning attachment, but you have to take off a fair amount of brass close to the neck, and I was having a hard time gauging exactly where to stop. I didn't want to end up with a donut on the inside and end up having to ream out anyway.
With the reamer, it's a quick and easy process. You're left with a neck that still has a very small amount of variation, but if you want to take 1/2 a thousandth or so off 50% of the exterior to even everything out, it's straight forward and less apt to cause issues.
As to the 21st Century tool, I've never used it, but it looks like a great unit. I've thought about buying one a few times, but the price always pulls me back. With the shell holder, I'm not sure which one would fit.
It's one of the things I like about the Forster tool: The smaller rim size of the 8x68S doesn't really matter. The .532 collett will shrink down enough to grab hold of the .512" rim (barely), and you can always order a custom collett from Forster if you want (I don't think I need to).
With all that said, if you can make an exterior neck turner work for you, then putting the case through an expander die after initial sizing then doing an exterior neck turn will work well.