There may not be much difference in performance between the two interfaces (since neither bus will be swamped with just 2 disks) but there is a vast difference in performance between 146GB disks and 500GB disks. Forget the interfacce and lookup the seek times and rotational latency of the two, that's what you should compare, e.g. 147GB 15K about 180 IOPS, 500GB 7.2K about 75 IOPS.