Saddles come in different styles for different riding disciplines and are made out of different materials, leather and synthetic. Some saddles can be easily adjusted with a changeable gullet and others can only be adjusted with a special machine.
Leather saddles can be re stuffed and re fitted to different horses, or when horse change shape through work or growth. Resale is also better on a leather saddle then a synthetic saddle, although normally the initial purchase price is higher.
If budget is a consideration, you would perhaps be better with a synthetic saddle then a poorer quality leather saddle. Synthetic saddles are very well priced and easy to look after. They can be washed down with a hose and are very light. This makes them great for children. They are also suited to pleasure riding. They can get wet on trail rides and stay out in the weather. Synthetic saddles can be harder to fit and they will not last as long as a quality leather saddle as they lose their shape.
The other consideration is an adjustable gullet. If you can get the style of saddle you like in an adjustable gullet this can be a real advantage. You can change the gullet size to fit without having to buy a new saddle. Non adjustable gullets can only be changed a very small amount by heating them up on a special machine.