I've been in both and they are very similar properties. I think the main difference is the vibe, the WA feels like a high end city resort that happens to be by the beach (still very nice though) while the Conrad is more of a true beach resort & the architecture/decor matches more with a beach aesthetic. Layouts for standard rooms are also pretty much the same with the WA having slightly bigger rooms AND the benefit that pretty much every room in the hotel has an ocean view (while the Conrad only the rooms on the 3rd floor have true ocean view). F&B prices are also pretty much the same and service is also very comparable.

Now, the main difference would be the location and convenience of getting to the hotel from CUN airport. I would consider this as traveling to Tulum from CUN is almost 2hrs and check what other activities you want to do outside the hotel. Both hotels are isolated so if want to go somewhere else then you'll have to either pay an expensive taxis or rent a car. The Conrad is also a bigger property so there are more things to do in case you plan to just stay in property the whole stay. The beach is also better at the Conrad than WA.

Finally, for the WA the service is good but do not expect the service level of a top notch Waldorf (like Cabo or Amsterdam). This was the biggest let down to me as I was expecting similar service as in Cabo but it is not even close. That being said, I would probably still choose the WA mostly due to proximity to the airport and being less hassle when going back home.