I’ve shown in synthetic with no issues in the past. As for the size if it’s a deep seat you won’t have much wiggle room on sizing. If it’s an open seat (flatter seat without the high cantle) you can wiggle in sizes a bit. I need at least a 17.5" deep seat (depends on brand and saddle) but ride perfectly fine in a 17" open seat. I have no where to try saddles where I live either and it’s a nightmare. We do have an independent traveling saddle fitter with used saddles of all types, sizes and prices and makes and it’s well worth the $100 to have her come out and sit in a bunch and try riding in them. She does a fitting on your horse and you and if she doesn’t have it she tells you what to shop for. Look around to see if you’re lucky enough to have one who comes to your area. Good luck!
Okay, I’m think I’m going to trial the Bates and then if that falls through, then I’ll go to the (easier to find!) synthetic wintec. I’ll post pictures if I get ahold of the bates!
Good to know that it the synthetic doesn’t matter. When I did barrel racing when I was young, no one cared as to what tack was used, as long as you performed well and managed top of the divisions. When I switched over to Hunter/Jumpers, I was converted into the whole you-must-absolutely-have-this-and-NOT-this mentality. So I’m happy to know that I’ll be fine with the wintec 
After saddle shopping second hand for both a dressage and a jump saddle in the past few years, and riding in lots of other people’s saddles, and watching friends and acquaintances go shopping new or used, here’s what I’d say.
Many people don’t know if their saddle fits them or not, and if you have a good sense of what that means for you, a good fit in your current saddle to compare, rode in a dressage saddle you liked, and a competent trainer to help you out, you are ahead of the game.
I think size would make some difference if you were comparing the same saddle with the same options, or if you were at the limit of one size, ie almost too big for an 18.
But I think that the shape of saddles from brand to brand is so different, that you might fit an 18 in one and a 17.5 in another, just like we all know there are shoe brands or jeans or breeches that “run small” or “run large” for their size.
Also one of the key things in fitting a jump saddle is if your knee runs over the block. With a dressage saddle, it’s more about where your butt sits in the saddle, if you are at the proper point or jammed forward or back. So you might find you can get away with different sizes in different saddles, depending on why you need the larger size in the jump saddle. For instance, if it was to accommodate along femur in short stirrups, that might not be an issue in the dressage saddle where your leg hangs longer.
So if your perfect test ride was in an 18, don’t order a 17.5 just because that’s what’s available. But if you are saddle shopping, expand your search to see what other brands are around. 17.5 is a very common size, 18 is much rarer (as I found, ending up buying two 18s).
As far as synthetic, I’m horrified by it :). But on the other hand, back as a re-rider and leaser, I ended up riding in synthetic saddles several times for a long time without realizing they were synthetic (despite being an astute and picky shopper for handbags and human shoes). I didn’t know the brands yet, and they just looked like ordinary saddles. One of my friends bought a thoroughgood because it fit her and her horse the best, and it is just fine. And it was in near-new condition for cheap cheap cheap.
So I’d say if the saddle fits you and your horse, that’s the best saddle for you, despite what it’s made of.
Looking back, I see you were comparing a Bates Isable and a Wintec Isabel. I think this is the same company, at some level, but different models of saddle with different company websites, and the saddles are not identical.
What do you mean, OP? I was a regional champion at third and a reserve regional champion at fourth in a synthetic Old-model Isabell. I’m very much a 17.5 due to my body but I can do a 17 in certain models. 18 never works for me but I can ride in it if necessary.
I used to ride and show in the synthetic Isabell. I loved it and wish I could use it on my current horse (he’s too short in the back for it). If you ride a nice test, it doesn’t matter what saddle you show in 
And my preference is to go up a half size if I’m unsure, but I don’t like to be locked into position either.
A friend got her bronze medal in a Wintec. Railbirds might notice, but the judges don’t seem to.
I got a Isabell synthetic for my youngster since my trainer’s fits him well. I am going to be getting something different eventually, as it isn’t as good a fit for me.
Yes, a 1/2 inch makes a difference. If an 18" fits you best (in that particular model), then you could ride in a 17.5 without pain, and it would be acceptable for a few rides, or if you didn’t have a choice, but I certainly would not go out an buy one. That being said, each model is different. Just because your current 18" fits you doesn’t mean that and 18" will definitely be the best fit for you in EVERY model. So it would be worth trying to get a trial period or one of that model to try and see how it works for you.
