If room size is what is most important to you than Hilton AMS is probably a better choice.
If you are going to actually want to explore most of central Amsterdam, than the Doubletree is a much better choice as it is much easier to get to anywhere in AMS from Centraal Station than where the Hilton is. In addition, Diamond Guests are allowed to get breakfast in the restaurant which has a much larger offering than the lounge. I can personally vouch for this as I stayed at the DT in late July-early August.
The room size did not bother me as I either was sleeping in my hotel or enjoying what Amsterdam had to offer. I would not hesitate to stay at the Doubletree AMS again. In addition, if you are going when its warm, they have an amazing rooftop bar/lounge that gives great views of all of Amsterdam.