I have stayed in the North, but not at the Westin. I have stayed at Iberostar, Grand Park Royal, Coral Princess, Casa Mexicana, El Cozumeleno, Casa Del Mar, Cozumel Palace. Most of these we have stayed two or three times. I have also dove with Salty Endeavors, from locations North and South.
Henry has had the Mental Floss and Living the Dream for many years. I hear they just added a third. I have also been diving with 5 or 6 other operators. All of the other operators were in "fast" boats, with the exception of Aqua Safari, which I wouldn't use again. Of the "fast" boats, the other ops had twin engines, which were a bit faster, as well as offer redundancy if there were a problem with the engine. While I have never been stranded at sea, it has happened and you can find threads about it here on scubaboard. While Salty Endeavors boats are faster than the large diesel boats, they are certainly not faster than the other ops using "fast" boats, and slower than the other ops I have used with twins.
Next, there is no comparison of North vs South. It is way better to be at Cozumel Palace, Grand Park Royal, or south for the diving. We dove with Pepe Scuba when we were at Coral Princess, and the op was good, but the boat ride was 45 minutes to an hour to get to the main sites in the south. When we dove with Salty from the north we would taxi to the marina and leave from there. That made for more time in cab, but less time in rough water. I generally prefer the 15 minute boat rides from Coz Palace or Grand Park Royal areas. I take photos and that gives me just enough time to get my camera rig ready, suit up, and roll in. When we stayed at the Iberostar we were always rushed because sometimes it was less than 5 minutes to get from their pier to the dive sites. That isn't enough time to get scuba gear on and camera ready, so I was usually the last one off the boat. Not a huge deal, but I prefer having time to get ready and not be rushed to get in the water.
My go to resort when traveling with others, divers and non-divers is Grand Park Royal. You can walk to town if you like a bit of exercise, the boat rides are short enough, and you have the Mega and Chedraui close by. Cab rides are cheap, and the resort has activities for most. The food isn't the best on the island, but I thought it was easily as good as the Iberostar, with a better location. Also, Iberostar fogs for mosquitos every morning, which I didn't enjoy. We like to get a walk in every morning and we would have to get out very early, or risk getting caught in the insecticide fog they spray. Also, they use reclaimed water for irrigation of the grass and plants. I appreciate the effort to conserve water, but the reclaimed water smells like sewage. The walk back from breakfast is always accompanied by a sewer smell, which I could do without.
For these reasons, our 'sweet spot' is the middle to lower third of the island. Coz Palace is nice, but expensive. Casa Del Mar is very cheap, but lacks some amenities. GPR is a good value, and best when we bring our adult kids to dive.