Product review (3 stars): B is the better filter according to other sellers
A direct quote from where I normally buy our household filters:

The factory original LG LT600P (5231JA2006B) Filter is used in refrigerators by LG, Kenmore and others.  The LT600P filter creates better tasting water for drinking and making beverages as well as clean, clearer, healthy ice.  The 5231JA2006B is interchangeable with the 5231JA2006A filter - but it is certified to remove cysts and other impurities that the "A" filter is not rated to remove.