Pat, it's just as Virginia says.
Best Press is really more of a Sizing product which doesn't stiffen fabric. It's great for fabric that will be cut into pieces for quilt piecing and that kind of thing, BUT for embroidery you need stiffness. Also, the same goes for decorative stitching. Stiff!
Now about pressing Starched Fabric. It will not mess up your iron if you do it properly.
This doesn't work with polyester fabric, polyester doesn't absorb the starch.
Why starch messes up an iron:
As you can see this takes a little foresight to plan ahead and prepare the fabric because it takes some extra time. It's well worth your efforts. You'll notice a huge difference in the quality of your embroidered & decorative stitching items.
Now about Terial: It's a great product too. It also requires some planning ahead to prepare the fabric. Best results are to soak the fabric by either spraying it or soaking in a bowl. Let it air dry for quite awhile before you try to press it.
I like it alot for clothing because it is so stiff, BUT I don't like the smell of it at all, so for most of my embroidery I prefer the Starch method.
Also, I have problems with the Terial spotting the fabric, so it has to be washed out, whereas with Starch I don't have the spotting problem.
Hope this helps!