Mike - I have the 670MXL (7700MX) which is the version with the Dichroic color head. As has been noted. the column height and max magnification are greater than the 6700.
2 big differences for me are in the features of the dichroic head. The 670MXL has a reversible recessed lensboard, and the 6700 only has a built-in lensboard. The 670MXL also has an option for a fine focus control and a focusing extension cable. The extension makes it much easier when doing bigger enlargements, unless you have really long arms. On the baseboard with an 80mm lens, you should be able to print a 6x7 neg to almost 15.5x18 in, although at those sizes it might be easier to reverse the column and print off the baseboard, or on the floor.
I've only used the 670MXL for B/W, but I've been very happy with it. Very solid and stable, the controls and filtration dials are easy to use. Especially if you like to do split-contrast printing, the settings are easy to see and adjust on the fly. I've used the tilting lens stage to correct minor perspective distortion. It'd be nice if it went up to 6x9, but I have a 4x5 enlarger also if I need to do that.