You should also consider the time of year. March is notoriously erratic for weather and strong winds and high seas are not uncommon. So, my recommendation is to NOT make advance reservations anywhere that you can't cancel at the last minute.
If conditions are ideal when you land, a 3-day itinerary, for me, would look like this:
Arrive in Palm Beach area in the evening. The virtues of Palm Beach/Jupiter for megafauna and some decent wrecks and reefs has already been discussed. I love diving in this area and actually do most of my diving there even though I live in Ft. Lauderdale.
Day 1: Morning boat dives on a wreck then reef. Good chance of seeing lemon sharks on non-feeding dives (I am not a fan of feeding, but some ops do offer that) in March and the operators will know the best sites, also good chance of seeing turtles on reefs and Goliath grouper are always present. You just don't get these big animals in the Keys with the same reliability. Get any of the boats out of Jupiter or North Palm Beach
Repeat in the afternoon (this is nitrox only, by the way, as the sites tend to be deep), or, if tide permits, a macro/muck dive at blue heron bridge if you want something different and fascinating (not to mention 3 hour bottom times if you rent a steel 100).
This being said, in March, you could arrive in Palm Beach and be blown out because all of the reefs and wrecks are offshore, exposed and deeper, so high seas basically ruin the dives by making the boat entry and exit very difficult (and in those conditions even Blue Heron Bridge, while protected, can become murky due to stirred up sand coming in on the tide). So, be prepared to bail on this area if its windy when you arrive and go to Key Largo.
Continiung the "ideal" itinerary, after your day in West Palm/Jupiter, drive to Key Largo
Day 2-3: Take advantage of the assorted reefs and wrecks in Key Largo. For reefs don't miss Molasses and French. For wrecks, don't miss the Spiegel and Duanne. Lots of other great dives that the operators will put you on.
In general, Key Largo is more "reliable" in March because there are a lot of reefs inshore of the ones right at the edge of the gulf stream (Molasses, French, Elbow are the "outside" reefs by the gulf stream) and these inshore reefs can be very good and diveable even if the wind kicks up, because even if seas are 6' offshore, it might be just a 3' chop on the inside reefs.
The "ideal" trip would really give you the "best of the best" ocean diving, although there are lots of other great sites. If it is windy, though, its probably better to bail on West Palm and spend the extra day in Key Largo.