Sam, you put it very well. :thumbup:
The 78 is quite a bit larger, even though only 1/4 inch longer, and the springs are strong. The 68 is more pocket friendly and easier to open, usually about mid range in tension.
I might add that the action on most stiff or slow GEC's can be improved by flushing/oiling the joint and just general use; they will usually loosen up and have better snap. Some are slower to react to this treatment than others, but a tight knife is better than one that is too loose.