Congratulations!! Welcome to the wedding planning process. I don't know too much about Dream Tulum however, I am getting married in Moon Palace in a little less than two months. One thing to consider, Moon Palace is really Cancun, not Riveria Maya. I don't know if that makes a huge difference to you or not.
What we liked about Moon Palace is that it is only about 15 mins from the airport which is nice, b/c some of the resorts can take up to an hour and half to get to and when you get in, you just want to be there already! Moon Palace is a HUGE resort with a ton of options for restuarants, pools, golf, bars, etc. A lot of our guest are using this as their vacation, so I liked the idea that is was large so if they want to not hang out with the entire group, they can "get lost" and enjoy private time. The disadvantage of the large resort is A LOT more walking. Everything is very far apart and yes there are shuttles but you do have to wait for those which can be annoying.
One thing we considered as well was the compliementary things Moon Palace gives you if you have a large group booking. Since we will have over 75 room nights booked amoung our guests, we get to stay 7 nights free and free unlimited events. Which means we can have a welcome cocktail party earlier in the week that we may not have paid separately.
The downside of Moon Palace, the no vendor policy. There are ways around it though. However, their photographers are not that good but to get an outside photographer, you have to pay for a room night or two and that really adds on to the cost of a photographer. Additionally their flowers are outrageously priced as well as a few of their other "optional" services that you will need for a wedding.
Hope this gives you some insight. Let me know if you have any MP questions. Sorry I can't answer much about Dreams. Happy planning!!