You only get the URG if you have a confirmed booking and they walk you.

The 48 hour guarantee, just guarantees you a basic room at any hotel but you will PAY HEAVY for the room. There are a bunch of restrictions with it including: Special Events blackout dates, enchanted room upgrades doesn't apply, cannot use NUA's or any type of promotion or discounted rates, and you must pay the published price for length of stay. https://help.marriott.com/s/article/Article-22363