While full seats are "proper" for dressage, knee ptach breeches are not improper. Wear what feel most comforable to you. If they make you uncomfortable, it will affect your riding!
I really like TS breeches, but don't really like the price, so I only buy them on sale, lol. They are the best in the hot sticky summer that seems to last 10 months (or more) here in Houston. (Just FYI, today's high is 78!)
However, as a very short-waisted rider, I have to say that you must first find out which brands fit you and which don't. While Devonaire breeches may be perfect for someone else, I prefer to have my waistband NOT touching my bra. Even Ariats are a bit high waisted on me (I only even had fleece tights from them).
Go to a local tack store - hopefully one that has several different brands. Try on lots (even after you found a good one), and take notes on how the different brands (or even different lines from the same brand) fit. Pick one pair (or more if the prices are good, lol) to buy there, and you can always go online to see what they go for. I like to use google shopping. I can search for a specific brand or color, and it will send me to which ever site (Dressage Extensions, Equine Collections, Stateline Tack, even Smart Pak!) has what I want.