I agree, quality is very important. Good brands with Dressage models include Ansur, Sensation, Barefoot, Startrekk, Freeform, and Heather Moffet's Vogue and Fheonix saddles. Stay AWAY from Hilason, Maya, Sydney Saddleworks, Freemax, and anything else that retails or eBay
's new for less than $700. These "copy cat" saddles copy the look of quality saddles, but they are often poorly made with internal structures that are not suitable for use.
You also need to get a good saddle pad. Most manufacturer's will have recommendations. Skito pads, Equipedic, Haf, and Granduer (Grandeur?) are the most popular for use with treeless saddles. I have a Bob Marshall treeless endurance and I use a Skito pad.
There is a great group on Yahoo for treeless saddle information. Just go to groups.yahoo.com and search for treeless saddles. There's also a group just for Ansur brand saddles.