My advice: Sea World all the way. Even if it means cutting out a day at WDW, imo it's worth it. It's very interesting, educating, and, to me, utterly breathtaking. There's no other place in Orlando or most areas you can see all of those animals and fish all in one place. Needless to say, when I go to Sea World and then turn around and go to Epcot, it doesn't make me all that excited about Living Seas, LOL.
Plus there's a roller coast and water ride for adults/older children; not sure if they have a switching off policy or not, but if so, you and your husband could enjoy those rides as well.
The other good thing is it's not a place where you're always waiting in a line; aside from those 2 rides and if you decide to "visit" Wild Arctic (polar bear, beluga whale, and walrus exhibit) by air (aka motion simulator ride before heading to the exhibit) rather than by foot (same film, no ride), you're basically just walking in and seeing what you want for as long as you want.
First downside is the shows can be packed together timewise and spread out distancewise. Other downside? It is definitely NOT a rest day. You will walk a lot there. Example: when I was there last Thursday, and elderly gentleman staggered over to a bench and sat beside another elderly gentleman and muttered "I thought this was supposed to be the easy day?" Poor man. It's definitely not an easy day, but it's still worth it. I was only in the world for 7 nights/ 8 days and only had 1 off day. Next trip, we're adding a day and night and taking a 2nd off day. It's too much to do in such a short time without completely exhausting yourself and leaving you feeling awful the rest of the trip.
My best advice would also be to go around 1 or so in the afternoon or a little later. Most people go in the am and leave by afternoon, so you would time it to miss the crowds. Even going that late though, it's still a tiring day.