I think it has a lot to do with indv rider and indv horse. I have ridden in leather and wintecs over the last 8 years. Recently I was leasing a horse and used her Wintec Isabell and it was SO comfortable. I then purchased a horse in Dec, bought my own Wintec Isabell and found even with changing gullets it didnt fit my horse as well and I didn't feel secure in the saddle at all (also tried Wintec Pro on her). Ended up with a heavy (not crazy about how heavy this saddle is) used leather saddle (cost more then my new wintec isabell) that felt so comfortable I was giddy with comfort - and it fit my horse better.
I like the lightweight, stickness and ease of cleanup in a synthetic, but for me, it just wasn't the perfect fit for my horse, even with an easy change gullet system. I would purchase a used leather saddle over a new one again due to resale value (think buying new or used car values) and because it's already been broken in.
Also when you buy used anything, you often can try it out for a week - but when you buy new, no matter what you do, it's not new anymore (girth marks on billets) so it's nearly impossible to return.