There are several grades of ball bearings just like there are several grades of bolts, paints, etc., etc. Doesn't make them worse or better, just the right bearing for the right application. You wouldn't want to spend what it costs for a ball bearing on the space shuttle gyro for your wheelbarrow front wheel, and vice versa.
Grades are based on alloys used, # of balls, measurement tollerance (cheaper bearings tollerate larger difference in individual ball sizes than do expensive ones), seal type, etc.
SKF is a good brand and the grade they list for your rear end will do just fine.