Like every other brand of saddle, some horses it fits, some horses it doesn't. I have a TB mare, and the wintecs fit her quite nicely. The gullet sets make them more versatile, but still not guaranteed to fit every horse. I have used my 500 AP with the EW gullet in it on several several-hour long trail rides on a tubby little App as well. It just so happens that my TB and my friend's App are comfortable in the wintec tree (and both, though the App especially, are very loud about their opinions on comfort).
If there is a tack shop in your area, I would rec trailering your horse to the shop and trying on saddles the same way you would try on pants. If not, most major websites have some sort of a demo option to see if the Isabel fits your horse.
I personally love synthetic saddles, but I am less than impressed by the Thorowgoods that I have seen, and find the Wintecs to be a much better quality, however, I have not had any experience with the tekna saddles. Also, I like the more mid-range wintecs. Not the original, older styles, but not the super new saddles either. I would like a slightly used one, and prefer the Pro (I think the one Solo had??) over the Isabel for some reason.
Mojo, 1980?-10/1/2010. RIP Mo-man. I know your life had many bad times in it, and I sincerely hope I made your last few years here on Earth happy and comfortable, with as much love and yummies as your huge heart desired. I love you bud, have fun running in the pastures of heaven. Until we meet again...