There really is no difference in quality between glass or plastic message bottles. Glass of course is heavier and has the most authentic feel to it. Plastic is just as clear and will save you tremendously on postage. It really is a matter of personal preference. For a real inexpensive way to figure out which one you like try this sample pack fromcoloredsand.com
If you are handing them out, try glass for that extra WOW factor. If you are mailing them plastic is great due to the cheaper postage and you don't "have" to have a box. Some people get some of both and make sure that the real important ones,like the new mother-in-law, get the glass bottles. Either way you can't go wrong when you "Do It Yourself"