Maybe you already discovered this, but I believe the Serenity Bay cabanas are slightly cheaper (by $100+?) if that makes your decision any easier. That could mostly cover a brunch at Palo, or couple spa access for the duration of the cruise. We have 2 rowdy young boys so we had to get a cabana on Family Beach. We had Cabana 9 and loved it, although we had neighbors pretty close by on both sides. I think they are all pretty packed in but if they have enough vegetation they feel nice and secluded. There are some cabanas that are newer that supposedly don't have as much vegetation, so if that is important to you maybe search past threads to see if you can figure out which cabanas are newer. The furniture in our cabana was starting to get a little worn though it had a nice beachy vibe. (Maybe the furniture in the newer cabanas is is better shape?)
If I was sans kids and I had the option to try Serenity Bay I probably would do that only because if you are on a honeymoon, I'm guessing there is a chance you *might* someday want to have kids. So maybe try Serenity Bay while you can. Just my 2 cents. The Family Beach by the cabanas is SO not crowded and really nice though, so if you have the money to splurge they are awesome.
We had a Cabana at Atlantis also, which was about $220-250 or so. I'm not usually a person to splurge this much but it was also our first cruise and I wasn't sure DH would ever agree to go on another cruise. So I went all out and we really enjoyed it. While we were there, there was a downpour of rain and we were able to avoid it zipping up our cabana and huddling in the provided towels. Everyone else crowded in the bathroom or ran back to the resort. There was also a safe to put valuables, and attendants who could get lunch for you (included in the Aquaventure excursion price). Also, we waited for a while in the excursion line but when they were ready to get going, people who were going to be in cabanas were given higher priority boarding the buses and we were walked to our cabana by an Atlantis employee so no need to find your way around that huge resort. So while it wasn't quite as nice as the
Castaway Cay cabana experience, I still think it was a nice splurge.
Don't forget to bring a few dollars with you to tip the person driving you over to your cabana on Castaway Cay, the cabana attendant (who will probably be your concierge staff members) and the person getting your lunch at Atlantis. Supposedly you can also send postcards from the Castaway Cay post office, but we did not do this. If you do, you need to pay in cash.