JRY pretty much covered it. Dacron (polyester) is a totally different material.
The "fast flight" types come in two styles, and quality can vary a lot.
You have the 100% HMPE (UHMWP, HPPE), and you have HMPE/Vectran blends.
100% HMPE is normally slightly faster, but not quite as stable, especially in higher temperatures. This will vary depending on strand count, draw weight, the grade of the material, et.
HMPE/Vectran blends are usually a tad slower because Vectran is a heavier material. Again, it can vary based on the above plus the percentage of Vectran used. BCY materials use anywhere from 17% to 33% Vectran.
There are brand names of HMPE from highly respected companies like DSM. Their brand is "Dyneema". Dyneema comes in different grades. Original "Fast Flight" bowstring (invented by the founders of BCY when they worked for Brownell) is SK65 Dyneema. It has a considerable amount of elasticity when compared to the newer materials. 8190F and 8190 Universal are SK90 Dyneema--the highest grade currently available. BCY-X is SK90 Dyneema with Vectran.
BCY-X is my current favorite bowstring material.
Everyone has an opinion and a preference, but based on what we can measure (strength, durability, consistency, stability) then 8190F or Universal is the best 100% HMPE material available; BCY-X is the best HMPE blend.
Bowyers seem to be like everyone else--they have their preferences. I don't make strings for bowyers anymore, but when I did by far the most popular materials were Dynaflight '97 and 8125G. At the time, these were the best HMPE materials available--before 8190 and BCY-X were developed.
Chad