To maximize shore diving, either B or C since there's a lot more of it than on Aruba.
With a new diver, I'd go to Curacao as the entries are much simpler - most off a sandy beach and the few that need a dive dock to get out over the ironshore have one. Also most have faciities, often dive shops, snack shops etc. - you won't find any of that on Bonaire once you leave town.
The entries for the most part on Bonaire are physically harder - you're climbing over ironshore/coral rubble both on shore and often just off-shore. We did about 25 dives on either island, none on Curacao had that problem.
This is older but compare the entry photos to see the differences:
Scuba Shore Diving Region: ABC Islands
You will drive more on Curacao but the roads are mostly paved or improved dirt. Almost all the dives on Bonaire are off the main road so they are also.
Also if you want to get your new diver on an interesting wreck - it's the Tugboat in south Curacao - 17' deep.
10x the restaurants/activities on Curacao when you're not diving. And more people. Both get some cryuise traffic downtown also. The beaches are significantly better, there's a lot more of them and many are also dvie sites on Curacao if someone wants to sit out a dive - often at a snackbar/restaurant on-site. None of the dive sites on Bonaire have any sort of facilities once you leave town except for wherever the kite surfer food truck is that morning.
As was mentioned there's some casual theft on both islands at remote dive sites so don't bring anything you can't afford to lose. On Bonaire the instructions are to leave the windows down/doors open so you don't have to replace one. On Curacao I noticed some of the more popular sites had Curacao Tourism guards patrolling. And twice the on-site operators mentioned they'd watch our vehicles. Plus you can get tanjks/weights at any of them, on Bonaire you haul your own everywhere outside of the dive resorts.
On either island you have to go out of your way to find a drift dive and most of those are done off a boat. Except the far south sites on Bonaire - seemed calm at the surface but it was ripping at Vista Blue one afternoon. On a dive site map, those sites are marked advanced.