I am working on NCI-60 database for data mining purpose. In NCBI I found miRNA expression profiles (link). But for mRNA of each cell lines I found 2 kinds of expression profile array in NCBI (link) categorized as Affymetrix HG-U133A and HG-U133B. They also overlap many mRNA probes and their expression values. I can't understand which one to use or how? What is the difference?