Your dealer has little experience with Sidewinders and it shows. The Sidewinder is a very good product. Don't worry about the quality, reliability, or service life. Just maintain it as recommended.
There may be some concern from your trailer manufacturer. As some feel the re-location of the pivot point could be an issue.
My preference is for the auto slider. Because when I change trailers, I prefer not to need to buy a replacement sidewinder. You don't always need to. But it always works out that I would need to. It's that karma thing.
But the Sidewinder is an excellent alternative for peace of mind. Many happy customers on the forum.