There is so much more involved than just synthetic vs. leather.
There are a lot of BAD saddles out there. Really, really bad. Ebay is a great place to find a good saddle, but it's also a great place to find a BAD saddle. It is full of cheap knockoffs that will be uncomfortable for both you AND the horse. If the price looks too good to be true, it probably is.
I'm currently searching for a good saddle. If you look at my threads, there were a lot of great responses in there: Good quality BLACK western saddle? Help me choose a western saddle
I'm currently debating leather vs. synthetic. While I prefer the "look" of leather, I do appreciate the price and light weight of a synthetic. It also seems like you can "feel" the horse a lot better through a synthetic than a leather saddle.
Also, don't underestimate the importance of a good pad. Lots of good info here: Why are western saddle pads so thick?! Saddle pad recommendation - Western