since usb2 at 480 mbits/sec is faster than firewire 400 and out of my 4 macs only usb2 is common across them all, it's a no brainer.
Just a point of difference here. I've actually tested drives that have both USB2 and FW400 interfaces on PC's and Mac's with both interfaces. My own experience has shown a 20-25% difference on timed copies of data (BTW, FW wins the test). In order to do the test properly, you must have several large collections of files so that you don't get any benefits from the memory cache on a drive when you copy the same file over and over, but rather just test the throughput. I hear this same raw speed statistic when dealing with sales people at computer hardware stores. They read the box, don't really know the technologies involved and assume the numbers are right.
http://www.usb-ware.com/firewire-vs-usb.htm and many other references. Simply google, firewire 400 vs usb 2.0
In general, I've found that the WD drives are a crap-shoot. Several of my clients bought the Costco specials when they started selling a year or so ago. Firmware updates came out, but these clients did not know about them, and I ended up having to get the drives to work properly. My clients didn't save a dime after paying for my repairs. The only good news is that WD has responded several times with firmware updates, and if you know what a firmware is, and you know how to install it, you might fix the problem yourself. But that's not what they paid for.
Even after updating the firmware's two of these drives continue to just 'go-away' on occasion for TM updates (the drives work for a couple of weeks, then must be power cycled to get them back on the job). I suspect the firmware settings for power management is not really Mac compatible. Maybe putting a drive to sleep is OK, if an OS can wake it back up, but that would be a job for the OS, or a firmware designed to work with MacOS. None of the programs or firmware's on a WD drive are worth having, and can get in the way on a Mac. I've seen it happen several times on different model drives, on Firewire and USB ports.
Recent past history that now includes problems with their USB drives also suggests that WD is still playing with the protocols, and the drives will be a crap-shoot in the future, while other drives just keep working.
WD has tried to write firmwares to improve their drives, but MacOS is a changing thing, with new stricter timing and error checking routines common with each upgrade to a new OS. Very few drives from other companies suffer the drop-outs we see with these WD drives and it seems that the PC users don't really benefit from these attemps either. Netgear's ReadyNAS support forums also discourage using WD drives in a NAS, as the power management features can cause corruption and drive array rebuilds. YMMV - I'll stick to Seagate, Hitachi, Samsung, Lacie, probably in that order (although I don't have any Lacie drives in my shop).
These facts have been true since Leopard was released and the MyBook FW drives came out, and we endured another round of reported failures common to this brand when SL came out and USB issues started. For the foreseeable future, it's 'guilt by association' for me when making a drive recommendation.
