I am wondering what if any difference there is in the performance of these string materials. I have shot B50 but have not shot B55. I am interested not so much in speed but in the feel of the shot. I am wanting to try them on my ASLs. Any information will be very much appreciated.

I like B55 slightly better.
Another point, B55 doesn't bleed color as bad as B50.

I've always used B-50. When they stopped making it I changed to B-55. Only difference I've noticed is smaller diameter.

Shot both no preference

B50 stretches more than B55 (BCY55). I pre-stretch my LB strings with 63 kg (140 lbs) weight, at least over night, usually a couple of days. I have not measured the difference, perhaps 50 % less. After pre-stretching B55 also sets faster to proper string height on my ca. 75 - 85 lbs bows, single loop on upper nock and bowyers hitch on lower nock.

Note: Polyester, the chemical name for these string materials, is the only allowed string material in IFAA's Historic Bow classes. That's why I'm stuck on these instead of using Fast Flight or other more modern less stretchy materials.

I prefer B55 for all the reasons Pine and Lehtis posted. 

I'll add one more personal reason - the "spring" of Dacron (polyester) affords me some measure of shot "stability" that I typically find lacking with HMPE string fibers.

Thank you for all your responses.

B55 all the way. B50 is way too stretchy and b55 is plenty safe for non free bows.

B-50 is not being made anymore. You can still find a spool or two but B-55 has taken its place.

Brownell still makes and sells spools of B50 bowstring fiber.