I've used B-50 and B-55. When I bought B-55, it was labeled B-55 Polyester, when I bought the roll of B-50 I have its label says B-50 Dacron. I don't know what the difference is but I haven't noticed a difference in performance. As for the Flemish or continuous style string, there are a lot of opinions out there about that subject, I personally make the continuous style string, otherwise known as the endless string, because its the only string I know how to build, I haven't taken the time to teach myself how to make a Flemish string. As for shooting them, I haven't noticed a difference other than with a Flemish string you can twist or untwist the string to raise or lower the brace height of your bow, which can help with tuning your bow for good arrow flight and to make your bow quieter (a concern if you're hunting with your bow). I've fiddled with that twisting and untwisting to find that sweet spot before but once again, I haven't really noticed a significant difference, so either string works for me, I just use the endless because that's what I can make. An endless or continuous style string is not difficult to make, I learned how to make one from the chapter on bowstrings in Traditional Bowyers' Bible volume II. I've also used linen to make an endless string and it worked well. Hope this helps.
Aaron