I rode in a couple Wintecs about 7 years ago. Their technology may have changed since then (especially the addition of changeable gullet & CAIR), but in general, I did not like the Wintec.
1-The saddles I rode were black...but after a while, they faded to a dusty, grayish, boring color.
2-They just felt weird! Also, they had Wintec "leathers" which did not soften like leather and were VERY stiff and plastic feeling. When you ran the irons up and lunged your horse, they would slide down the "leathers" even at a walk and then bang against your horse's sides.
3-I too had the slipping problem. I played Fun Polo (beach ball and brooms) in a Wintec. I would be off balanced a lot in order to take a swing at the ball and the saddle would slide like crazy! I could imagine if your horse spooked or something that threw you off balance...you'd slide right to the ground with the saddle on your horse's belly!
The only benefit I see in the Wintecs are that they're waterproof (for trail riding or if you only have an outdoor arena) and the changeable gullet.
I just looked up the Wintec you mentioned on Dover...$625!!! You could get an amazing brand-name, European, quality saddle (lightly used) that will last forever on eBay
for that! Check out the older Crosbys and Stubbens. I just bought a demo-condition Pessoa for $850 and I only spent around $300 for my show-condition Crosby Sovereign PDN (plain flap). Or even check out the used saddles at your local tack shop.
If you're really a fan of the changeable gullet (like, for a young, growing horse), try a Bates..although, even used on eBay they'll run about $1000.
You'll be much happier (and secure in your seat!) in a quality made leather saddle...even if it's previously owned!