I haven't actually seen one but as far as I understand the aspide is supposed to be slimmer, less padding in the seat, and less padding in the nose, narrower profile, etc. Best bet is to go to the selle san marco site and from teh pictures you can get a good idea of the differences

Just road on the new Aspide this weekend, had the larger Azoto saddle previously. I liked the new Aspide , very comfortable, probably just as much as the thicker Azoto. Didn't feel much difference in the aero position. It feels about the same when you sit back on the rear of the saddle. It is a bit narrower but it fits me fine. They cut quite a bit of wieght off as well, I think70 -80 grams, if you consider this alot. Overall i like it , much sexier look as well. The titanium rails were good , with the old Azoto you could feel some flex in the saddle when you were forward on it. This may have been a function of the rails and the excessive padding. The new Aspide has cut down the padding, and the stiffer rails makes the saddle a bit stiffer as well, but I don't think the comfort has been sacraficed, overall I like it.