First, take this with a grain of salt since it's been 10 years since I did the Cay Sal route. But I'd say Cay Sal is better overall for sharks. Although the wreck of the Hesperus on the Northern Bahamas route has a lot of sharks on it.
I don't remember seeing any dolphins on the Cay Sal trip. But dolphins played around the boat both times I was on the Northern Bahamas trip. They try to let you snorkel with the dolphins on that itinerary. But wild animals being wild animals, it's pretty hit and miss. Both times I've done that trip we didn't get the chance to snorkel with the dolphins.
Both routes have some excellent sites. The Cay Sal route has a drift dive and they also dive some blue holes. I'd say the Cay Sal route also tends to be a little deeper.
The Northern Route has a couple of shallow wreck sites that are pretty cool. You can also go into shore on Grand Bahama and Bimini if you want.
You'll have a great trip either way. For me the next route is Cay Lobos next summer. That is I don't sneak off to Belize before that.
Hope that helps.
Luke