JRY has good advice. I've been making strings for a little over 20 years myself, most of that time commercially, but I've been a loyal BCY customer for most of that time also so most of my experience is with their materials.
I do study all the different string materials as best I can. Brownell isn't nearly as revealing as to what is in theirs as BCY is (BCY has it listed on their site).
Here's a basic rundown on the characteristics of the different materials. Consider there are lots of variables, from strand count to string build to draw weight, etc. Some of the following is opinion based on my personal experience, but most of it is fact and easily verified.
Polyester/Dacron: Safe for any bow. In my opinion the most aggravating materials to work with, because of the amount of elasticity. B-55 is made from a higher grade of polyester than B-50, but it can still be tricky. Felt handshock appears to be the worst with this type material. B-55 and B-50 are the only two bowstring materials I know of that are still made with polyester. Dacron is just a trade name for DuPont's brand of polyester.
HMPE/Dyneema: High Modulus Polyethylene, also known as UHMWPE (Ultra High Molecular Weight Polyethylene). Another popular brand name is Spectra, but there are lots of Chinese knock-offs that are not consistent. The strongest and most durable man-made fiber available. Currently the highest grade is SK90. Original "Fast Flight" (invented by the founders of BCY when they worked for Brownell, Ray Browne and Bob Destin) is SK65 Spectra. The original formula is still available from BCY as 652 Spectra.
This material continues to evolve and improve. Many very popular bowstring materials are still made with 100% Dyneema: Dynaflight '97 and Formula 8125 (SK75), 8190F and 8190 Universal (SK90). The fastest (lightest) material, but prone to a little stretch/creep when exposed to high temps, especially in lower strand count strings and/or higher draw weight bows. Felt hand shock seems to be greatly reduced with these materials, at least in some bows.
HMPE Blends: Every blend I know of on the market currently contains Vectran (in different percentages). Vectran is a Liquid Crystal Polymer (LCP). It's in the same family as Kevlar, and similar to Kevlar you can't make a string material from 100% Vectran. Flexed over and over in the same place (i.e., the nock point) it will eventually break with no warning. However, properly blended with HMPE it makes a very consistent and stable bowstring that is only slightly slower than a 100% HMPE material (due to the Vectran being a heavier material). In my experience, these materials are also quieter. Some of these are BCY-X (17% Vectran, 83% SK90 Dyneema), 452X (33% Vectran, 67% SK75 Dyneema), and 450+ (30% Vectran, 70% DK75 Dyneema). The first blended material was invented by one of the founders of BCY, Bob Destin, who is also the co-inventor of the original Fast Flight string material. Like 100% HMPE materials, felt hand shock seems to be greatly reduced with these materials, at least in some bows.
Hope this helps. I love to talk string materials!
Chad