As a two legged pony, I don't usually have much occasion for my saddles, which is a shame since I really love them. My favorite saddles are those made for small bio-equines. Aside from the realism they convey, these saddles are quite comfortable on my back (many of them are very well padded).
Additionally, I have found that if one uses a real jumping saddle, one can also use a real girth (a dressage girth works surprisingly well, and many girths for horses are also very well padded and comfortable). Using a girth adds to the realism of the pony, which is something I especially like.
Although most saddles made for ponyplay will not be interchangeable between two and four legged ponies, I've seen one saddle that looks like it can be used comfortably for either type of riding.
Saddles can be used on show ponies as well as riding ponies. On a show pony, the saddle can be there just to "look pretty," or it can be functional. Take into consideration the use of the saddle when purchasing one. A nice looking saddle might not be appropriate for use in riding.
A saddle used for riding should fit the pony well and distribute the rider's weight evenly over the body of the ponygirl or ponyboy. It should also be comfortable and somewhat padded and be made of sufficient quality such that it will easily withstand the stress of riding. This is part of the reason I prefer saddles for real ponies and horses: they are designed to withstand the weight of a rider and distribute such evenly over the body of the horse.
However, a horse's body is obviously not the same shape as the body of a ponygirl or ponyboy, so I would suggest experimenting with different saddles to see what is most comfortable. Many tack shops will allow you to take a "test ride" on a saddle to see if it fits your needs (you take the saddle for a few days and test it out, and if you don't like it, you return it). If you decide to purchase a saddle made specifically for human ponies, there are a few good choices and custom designed saddles are also a possibility.