I just got back from Marathon Key a week ago and I’ve also been to 3 of the Hawaiian islands. For a big event like your 40th anniversary I’d suggest Hawaii if you haven’t been there before.
I’ve done water activities at both. In our 3 trips my wife and I both agree that the Pacific waters, at least strictly comparing Hawaii to the Keys area, are much rougher. We went out fishing for 8 hours and were a little woozy the entire time. We also did a dinner cruise that got a little rough as well.
Compare that to the Keys, where I’ve been 15-20 miles offshore and it seems just fine.
Weather-wise, a week ago in the Keys it was about 80-85 every day we were there. The news did seem to say a lot that each day was approaching record highs each day so maybe it was a little unusual but the weather seemed great. We kind of hand-pick the timeframe we go somewhere warm each year so that it’s early-to-mid-February where you’ve had enough winter, get a good break, and don’t have as much winter left when you come back.
I don’t know if you’re interested in snorkeling or not; I never was but my wife really wanted to try and now we have our own snorkel gear and it’s pretty fun. We used a service out of Captain Pips in Marathon Key that charge $28 (plus taxes of about $10) per person to take you out to Sombrero Reef, which I guess is the world’s 3rd-largest reef. Snorkeling was good there, just had to dodge a few jellyfish. The day we went snorkeling it was just a tad choppy that far out but the boat captain said it just gets better and calmer as the year goes on.
I’d be glad to provide more info on either location if you want to PM me with more questions. Hawaii does get my vote for the big occasion, it’ll just be more spendy to travel and stay there. There is just a ton to do and see in Hawaii. I’d also say you’re on the right track with your two options; we’ve been to the Dominican Republic, Bahamas, St. Martin, and Anguila as well and while my wife is more interested in those foreign adventures I’m always much more comfortable getting the warm-weather fix while still using American currency, knowing I’ll get American healthcare where I can use my insurance, etc.