Well .....one noteable difference is bugs, specifcally silverfish, will eat starch and damage the fabric that has been treated with it.
Bugs do not eat sizing!

I prefer sizing. It removes wrinkles and creases much better than starch or water.

I also prefer the Magic Sizing it smells better and as others have stated do several applications if you want the fabric stiffer.

What is "hoop burn"?

When you embroider, sometimes there is a mark left where the hoop was. That's hoop burn. Spraying the "burn" area with Magic Sizing will remove it.

Thanks everyone, for the tips. I've never heard of Best Press but may check that out next time those Hancock's coupons come out. In the meantime, think I'll buy a can of Magic sizing and check it out. Thanks again ;)