Thought I should share my experience as someone who has bought the XD II with a 5400rpm drive recently. For those of you planning to buy a 5400rpm drive, you may want to reconsider if you are only going to use it as a portable drive.
Was just doing the math... USB2.0 = 480Mbps maximum transfer rate. This translates to approx. 468.75KBps. If I am not mistaken, even a 4200 rpm drive has a maximum interface transfer speed of 100MBps !!!
Even if you look at the media transfer rates, it's only 20+Mbps diff between 5400/4200rpm and the seek times are exactly the same!!! Well, I guess you get slightly better response in terms of latency, but if you are hoping that getting a 5400rpm drive means faster transfer between your memory card and the drive - forget it.
So I have been suckered (but I insisted even though the shop keeper warned me so ). Hope this helps you make a better decision.