Some random thoughts...
I have been to both a few times both on a land vacation and off a ship both working and as a tourist.. All the above responses are good except the one that said Key West is a grave yard of coral. Maybe it is where the cruise ship ops dump you but the same could be said about The Bahamas but that's another topic. In the end, it depends on how much time you have and who you go out with and where they take you.
The shark dive is fun to do once. Consider it. Nassau is kinda boring on land. The ships dock pretty far from the dive boats so doing it with the ship is a good idea as it will include transportation. Avoid the people who want to "take you for a dive" on the dock when you exit the ship. I have had friends who have had some scary experiences that way. Topside, Nassau is far from my favorite and I would never pick Nassau as a dive vacation only spot. Few would. I would however pick Kew West (and have) as a dive specific destination just because the topside activities are so good. The diving is just okay for the most part but better than Nassau typically IMHO.
Key West diving for coral and sea life is typically better but so is Key West topside. The Dives on both KW and Nassau from the ship both tend to be very "cattle" like experiences where a bunch of people are thrown into the water close to land in a beat up spot with very limited bottom time. IMHO if you have time, use a private op in KW should you choose it and do some searching on the Keys section here to pick a good one. The private ops will typically take you to better spots with fewer people but time is a factor here and sometimes transportation. But again, time is an issue because that ship will leave you. I have seen it done many times while working on them.
The reality is that neither Key West or Nassau is known for world class diving. Sure, it can be decent but it just won't be great. That said, when I had 0-24 dives both were amazing.
You know, I gotta change my mind here. If I were you, I would do the shark dive through the ship and walk around Key West, especially if you have never been to Key West. It's charming in a Key Weird way while Nassau is just kinda grungy near the ship. Key West can also be "taken in" in one day for the most part.