There was another thread recently where someone asked the same question you are asking.
The new DX6 vices have a design where the screw pulls the jaw instead of the old push screw used on the D675 and D688. Additionally, the DX6 is narrower, has bolt holes thru the bed, and is not compatible with the swivel base that is used with the D675 and D688 vises. I believe the vise jaws are the same on the DX6 as the old style vises.
I just bought a new vise and decided to stick with the D688 design for three reasons:
I don't like the bolt holes in the bed of the DX6 vise because I will never use them and they will get filled with chips. The presence of these holes seems stupid to me. Why wouldn't you just use the mounting ears on either side of the vise?
This vise is going to be used on a horizontal mill and I want to use my existing swivel base which won't work on the DX6 vise.
I never found any issues with my other D675 and D688 vises so why change even though the pull screw design seems superior?
