I'm partial to Kona, but I've done a limited amount of diving on Maui too and it's got some good things. I like the ends of Molokini, nice currents with sharks the times I've been there. I've liked the diving on Lanai, it's a lot like Kona's southern sites structure-wize. In both cases I felt the coral cover didn't match the reefs in Kona. The near shore Maui diving I've done (only 3-4 dives - red hill, the tank, old airport and another) was unspectacular.
So keeping in mind my Maui experiences are limited, I'll try to make a comparison.
Maui: More (and bigger) turtles. Potential for more sharks. Nice fish. Decent reef. OK viz. Long and potentially rough (tradewinds made for pounding on the way back every dive I've ever done there, don't know how often that's the case) boat ride.
Kona: More and healthier reef. Nice fish. Lots of good sites with shorter rides (unless taking medium-long range trips) with all day diving most days (tradewinds blocked by nearby volcanoes). Better viz nearshore. Deep water and shallow water at nearly every dive site. The manta dive.
As far as seeing dolphins and large life underwater, to some extent it's kind of random chance. Not sure about Maui, but Kona has one spot that divers have a good shot at dolphins 2-3 days a week, and a couple spots that dolphin sightings are not totally unexpected because they're on drop offs the dolphin pods pass near shore.... it's still no guarantee. The other day we scored a trifecta of sorts on one dive... Whaleshark, manta ray, and dolphins next to the divers on one dive - rare, could happen anywhere in Hawaii. Manta sightings during the day are not uncommon in Kona.
Both islands have good diving, it'll be tough to go wrong.