i wouldn't say i'm a pro....but my tendancy is towards Western Digital...mostly because they have the best warranties and easy, no hassle, fast RMAs.... the last time i RMA'd a drive, i returned a refurb 320GB and received a 500GB in return, SCORE!....and i've heard of that from others too, especially if you've returned drives in the past....so it is possible that you may receive a 1TB in return.....typically it depends on whatever they have on hand....
EDIT: which did it fail--the quick test, the extended test, or both?
EDIT 2: my sister's HDD (a WD320GB), is also currently failing both tests, but it functions well enough for her....her's went bad because her 3 yr old jammed 3 discs into the wii... ....after it went sour, the drive would only load data onto or format the first 100GB befoe erroring out--and slowly like you described (though not as slow: about 2x slower than a normal drive).... after running a CHKDSK scan in windows, the rest of the drive was freed up for data again...but it still loads slow and still fails the TESTS in DLG....it's out of warranty, so i told her to live with it until it dies.... ... ...then i put a big ole' piece of duct tape across the disc slot of the wii! ...damn kids.
(the last RMA'd drive i spoke of (above) went bad because of the same thing; that one was completely hosed though. can't explain why a bunch of discs in the slot makes the HDD physically go bad, but it's happened twice now to some extent....)