As a rule, I decide which weight to use, according to the thickness and length of fibers of the towels. A thin kitchen towel would only need a light solvy. An average bath towel may need a light or medium solvy, but on a thick, plush towel I would use heavy solvy. This is just my method.
Angel
I buy the bolt at Joanne's and use the coupon. i only use solvy for the top, because it tears away if used for the hoop. sad experience!!
I use regular solvy as my topper. I use a washaway vilene stick then stick the towel down then put the solvy on the top. I baste it down when I baste the towel to the stabilizer in the hoop.
I am guessing you are in the USA or Australia, ummmm what do you term as regular, here we only have light medium and heavy. And I have never seen vilene stick. I wonder what that would be here in England.
Regular solvy would be the light weight one (it is just called Solvy in the US - then there is Ultra Solvy which is ticker/heavier solvy). Vilene is the fibre type washaway (it looks like fabric rather than a film like Solvy) and it comes in a sticky and non-sticky version. The non-sticky is widely used for free-standing lace. I use the sticky on most everything I do because I use the "hoopless" method (where you only hoop the stabilizer and not the item).