I have both meters, the 508 is my backup. I use the 558 in aperture priority mode as a spot meter and can access the half stops(like 6.3, 13, 19, etc), while the 508 does not seem to be able to do this. Makes things really difficult for me when I have to use my backup meter.
I just double checked and I still can't get the 508 to read the between stops, and I haven't been able to figure out how to use the little number in parenthesis after the f stop. Comparing the two meters against each other the 508 seems to read scenes as needing slightly more exposure than the 558, but I suspect it's that decimal reading I'm not sure about or how to apply to the exposure time that throws the readings off.