For a young horse, I suggest anything that fits correctly! Unless you are competing in English either a GP or dressage style is fine for flat work.
I've ridden youngsters in anything from County, Keiffer, Stuben, Parker and the list goes on, it all depends on what fits, nothing else really matters.
If you are purchasing new, try a few. Once you decide then get a professional saddle fitter to fit it correctly, and remember that with more work your youngster will change shape so the saddle may have to be fitted again at around 6 months or so.
One of my previous hacks had her saddle changed about 6 times from age of 3 - 6, her shape just kept on changing and developing so much.