archonix, you'd see better bootup and program-startup times from the 120GXP array due to IBM's faster seek times. Meanwhile the WD drives are faster on moving large amounts of data. If you insist on booting from a RAID array then the 120GXPs would be the better choice, while the WDs make a better array for storing large data files, like audio/visual stuff.
Having said that, if I had to pick ONE of the above drives to boot off of, not an array, I'd still choose the WD and sell the IBMs as soon as possible. 120GXPs are more reliable than 60GXPs and far more reliable than 75GXPs, but they're not beyond suspicion. Mine have made unpleasant noises on occasion. Meanwhile WDs seem to either have problems right after you buy them, or not at all -- though the relevant behavior of the new JB series remains to be seen over time.