First of all, the following is just my viewpoint as a vacation diver.
My suggestion would be that if you have access to a dive shop at home or can find an instructor with access to a pool (maybe a local school) in your area, have your niece take OW ahead of the trip. She won't be rushed through, will learn a lot more, and could then do the checkout dives and enjoy other dives afterward wherever you wind up going. I can't say that all resort dive ops are going to rush a person through the course with just the basics, but I've seen it - people are only on vacation for so many days. As I think back, I was glad to have had time to soak everything in over several weeks as opposed to a few days - also nice to only worry about completing the checkout dives on our trip to Grand Cayman later that year. It's possible that if your niece excels in OW and completes the checkout dives (and money isn't an issue) she could then take AOW with you, but again, how much bookwork does a person want to do on a vacation?
As far as you and your husband are concerned, the same will be true for AOW training at a resort dive op as far as it being a rush job. You will have bookwork/test to complete before/after each of the five required dives and most likely have no training in water beforehand. Speaking for myself only, when I finally got my AOW, it was at a resort dive op but that's how I wanted it. I was just in it to get the c-card. My local dive op guy even said I should get AOW on a trip since all of my diving was in clear (most of the time), warm water. The course with him would have included a quarry dive (for the deep dive) in 40º F water at 60 ft.
Clive at Dive Aruba gets recommended here often.
Regarding your choices for your trip, Aruba didn't have the greatest visibility (month of June) and many dives are drift dives. A note about Clive. He's not a resort op, but offers OW, completion of OW checkout dives, and AOW. Unless he's changed his system, he goes down with divers for about 15-20 minutes, then goes back to the boat and picks you up where he thinks you will come up - maybe not the best for a new diver. Don't think he would do that though on training dives.
Cancun area covers a large area - just south of Cancun airport is Puerto Morelos with many AI's and some decent diving. Dive In Puerto Morelos is one dive op in the area - close to the pier.
Have been to a couple of the other locations but a different part or side of the island, so can't comment.
Agree with
@diversteve about Grand Cayman if that can become a possibility on your list. No real AI's (meal plan can be added at Cobalt Coast) but can pretty much guarantee calm, clear, warm water at most sites along 7 mile beach. DiveTech is also one of the great ops in GC.