I wouldn't mix B-55 with B-50 or B-500. My BCY price list also has them listed at the same price.
B-75 and PENN 66 were both taken off the market several years ago, because they broke with no warning. I remember that debacle all too well--I got called pretty much everything but smart and honest for raising questions about them when another stingmaker (no longer in the string business) was touting them as the best thing since sliced bread.
If I recall correctly, PENN 66/B-75 had a little less stretch/creep than B-55, but I think B-55 is going to be a lot more reliable.