Regulars to the forums can guess what I'm about to say.
First, I recommend staying away from all Seagate drives. Seagate are not known for their reliability, and of all the Seagate drives I've ever had or has been owned by friends and clients, they've all died by now. On the other hand, Samsung and WD drives are far more reliable and has better performance.
Secondly, 2TB have less performance than 1TB drives, so I generally advise against using one as a boot drive (however, if it's a secondary drive then it won't make any difference). A 1TB Samsung Spinpoint F3 or WD Caviar Black are great boot drives (if you can't afford a SSD) and then getting a 2TB Samsung Spinpoint F4EG or WD Caviar Green makes for an ideal setup. Personally, I prefer Samsung to WD, since Samsung drives are usually cheaper and they have the same level of performance.
If your choice is only between WD Caviar Black and Seagate Barracuda XT, then I'd always go for the WD drive.