Sorry it's taken so long to reply. I actually had to think about this for a while! I think the main differences between Sanibel and SOWal are:
Sanibel has plenty of casual, family friendly restaurants, Sowal does not. This is fine if you don't want to eat out with your kids, but if you do, you have to drive many miles to the Destin area to find a good casual kid-friendly restaurant.
Sanibel, being an island, was more self-contained, and we felt totally comfortable letting our kids ride their bikes down to the DQ or 7-11. The main commercial drag is about a block away from the beach road, so there's not much traffic on the road that runs closest to the beach.
The water is just as pretty at both places. Sanibel's beach was very flat, and the water was very shallow--knee deep---a great distance out. Very little surf (when we were there). Great shells! Sowal better for swimming and boogie boarding.
Sowal has a lot of single-family houses, so there's more available to rent if you have a big group. Sanibel's beachfront properties are mainly condos. Most have at least a pool, and many have tennis courts.
Sanibel's twice as far away as Sowal!
All in all I love both places, but I wish Sowal would have more of the casual feel that Sanibel does. (Like it was in the olden days!)