BTW, saw the WD 1TB drive at BB, but its not at my nearby store. Some people seem to think a 32 cache is best and that the WD is slower, but I guess its a small thing in the bigger picture.
If you read and trust the reviews, the conclusion is the Hitachi is definitely faster and the WD is definitely cooler. Now the questions for use with the HR20 are not so clear:
1. Is the Hitachi's faster performance likely to in any way make the HR20's use better than the WD's, or is the WD "fast enough" to be everything the HR20 can take advantage of. (IMO, probably yes, the WD is fast enough. Reviews indicate its likely to be at least as fast or faster than the internal drive that the HR20 ships with.) Now in my case, once I moved to the Hitachi, my whole HR20 experience improved - everything got smoother and zippier. However, I may well have had sort of a dud internal drive. Your mileage may vary.
2. Does the cooler temp of the WD translate into longer life ? Thats the big question. Its not so clear. Intuitively you'd think yes, but there was a big study earlier this year (using the drives in the Google data center), that surprisingly did NOT correlate drive failures to drive temperatures. (This study is easily found with an internet search.) That said, I certainly think running cool is a good thing.
The Hitachi drive is extremely quiet - it is the quietest hard drive I have ever owned. (And I have used MANY MANY hard drives.)
So which is better: I dunno. I like my experience so far with the Hitachi, but the WD also looks very interesting to me.
I think you can't answer this for at least a year - with some long term experiences - and even then I think the most likely answer will be that they are both good choices.