Tac-7: larger 'muff', more padding, single 9v battery, one on-off control.
Tac-6: smaller 'muff' (good for long guns), not as much padding, two AAA batteries per side (if memory serves, they last a while), and a control on each side.
I have both, and usually prefer the 6's. My wife can't stand the 6's, and refuses to wear them unless she is shooting her HBAR.
A previous poster mentioned botech had 6's at $115. That is a heck of a deal...I have been recommending Cheaper Than Dirt, at $125 or so (ten dollars more for the 7's).
Alex