The saddle should both fit you and your horse's morphology and habits. It is always better to have a professional coming to see you and your horse, he/she would check your horse morphology and overall condition, take measurement, would then let you try saddles, see how you feel in them and which one would be more suitable for you and your mount. If possible and depending on your budget, you can choose to have a tailor-made saddle, which you perfectly be adapted to you and your horse.
Depending on the size/ fit for your horse as well as a comfortable seat for you, you want to fit both correctly. Other than that, I cannot tell you exactly what to buy, but asking a professional to look at your horse and let you know what would be best is going to be your best option!