Did a little research. Compare the weights of white B-55 and white B-50, you should see practically no difference. If anything the B-50 may be a little heavier since it's a slightly larger strand. With bright colors, B-55 is probably going to be a tad heavier since the colors are brighter/more vibrant. That requires a thicker coating (the colors are a coating, not a dye). Not enough that it's going to make squat difference in performance, but I like to research stuff like this. Again though, if you want to squeeze out that last 1/2 fps, stick with a solid white string (and minimal serving). That holds true with any material.
Chad