The 25 has a weighted keel. The 26D & S have water ballast and are easier to tow. (about 1800 lbs + trailer weight) They take on about 1200 lbs in water ballast for stability. Not to be confused with the M or X which are motorsailers.
In shallow waters like I have in the bay, I like the S because if I drag, the board will just pivot up. The older D models are slightly faster because with no cut out in the hull for the swing board, there is less drag.
The D has a daggerboard, the S has a center board (swing board - S)