Here's some infor from a previous thread courtesy of Rod(NH). Hope it helps.
"In my opinion the vinyl ones are preferable but I suppose it's all in what appearance you are looking for. The cut vinyl decals are usually just for the main hood lettering. The letters are individually cut and there is both a backing and a topping to keep them in proper arrangement until they are applied. They are thicker but there is no clear mylar between the letters or around the entire decal. When the vinyl ones are clearcoated over, the edges of the letters are softened and are blended into the surrounding area. that gives a look more like they were painted on. With the mylar ones (which are one piece), there is a square telltale clear around the decal and between the letters that make it more obvious that it is a decal. I think most vinyl sets will include mylar decals for all the smaller ones, limiting the vinyl type to the main hood decals. At least that was what the vinyl decal set for my AC-B was like. The application procedures are different so you need to get the correct information, especially for the vinyl type. Yes, the vinyl ones are more expensive, but worth it in my opinion. Originality would probably dictate the mylar type but then again originality would also dictate an alkyd enamel paint, not modern automotive urethanes that are far preferable in fade and chemical resistance."