Why those two?
You gettin em cheap or something?
Of those, I know that K&M offers a carbide reaming/turning mandrel, and has an expander die system. I don't remember if K&M has different angle cutters or not. But with a K&M, it's easy to view the cutting, and it's easy to clean(being an open design). Some folks don't like that K&Ms are so small.
They are all easy to adjust.
The whole eccentric mandrel adjusting is insignificant as a benefit really.
I don't like either. Sinclair has the best 'system' IMO.
Neck turning is not a no-brainer-throw it on a bench-and turn em all,, like trimming.
Often, it takes a strategy and the more options in your SYSTEM, the better.