WD1003FZEX is one platter, WD1002FAEX is 2 platters, single platter is better and 20% faster.. both have a 64MB buffer but the single platter's Transfer Rate is more efficient:
WD1003FZEX; Buffer To Host 6 Gb/s Max & Host to/from disk is 150 Mb/s (Max) vs the WD Black WD1002FAEX; Buffer To Host 6 Gb/s Max & Host to/from disk is 126 MB/s (Max).