The Hobie catalog has them with different catalog numbers (where the rudders and rudder castings for example are listed with the same catalog numbers for both the 17 and 18). Also all the individual parts that make up the mast have different m=numbers on the 17 and 18 such as the luff track.
Also the mast on the 18 is 28'1" and the 17 is shown as 27'7". but the length may be the only difference.
I know this didn't help much but good luck anyway.