Originally Posted by Global Adventurer

On site has worked for me much better. And you also get a better feel for where the room is. And they can provide tidbits of information on why a certain room might be a better choice, especially in hotels that have different types of rooms to upgrade to.

I've been upgraded on the app, and on arrival upgraded again to another type room.

However, if I have a late arrival, I always try to get upgraded via the app because an upgraded room most likely will be sold out when I arrive.