The 73 is a "square end" knife, meaning the bolster is square and flush with the flat part of the tang of the blade. Really kind of neat. The 73 also has a small "hump" in the middle of the blade well. The 74's additional length comes from the bolster being of normal curved shape and that is where the extra length comes from. The 73's that I own have a little stronger springs than the 74's that I have and I think this is generally true. Although the 74 is the longer of the two by about 1/8" when closed the 73 is the longer of the two by about the same amount when open. If we are talking about the drop point blade I would come pretty close to telling you they use the same blade for both but not absolutely sure about that. When it comes to deciding between the two for me it would come down to which handle material it was in and also how it was embellished, lined bolsters and such. I have a weakness for smooth oiled bone and therefore won't miss the 74 with those that's coming.
Take care,
Rick