We bought our first TT used to try out and see if we liked the lifestyle. Needless to say, we loved it, and soon were shopping for our first new unit. I was like you and wanted to find something that might be a bit better on fuel. We purchased an "ultralight" trailer based on that assumption but were disappointed when we found our milage didn't change at all.
The ultralight trailer was 26' and had a dry weight of just under 4,000#. We recently took delivery of a new trailer, and this time bought one with the features we wanted instead of worrying about the weight. The new trailer is 27' with a dry weight nearly 3,000# greater than the ultralight. (6,880#)
Guess what happened to our milage while towing? Nothing changed! I am kind of anal about fuel mileage and monitor it closely. We just returned from our first major trip with the new trailer and my average MPG was 9.5 for the entire trip. Virtually identical to my long term average with the old trailer.
Buy the one you like the best and meets your other needs. I am convinced that ultralight in TT's is nothing more than marketing and a way to make them cheaper.