Well a year ago I would have told you to buy the Samsung because in my opinion they are better drives and WD no longer allows the end user to be able to use a software tool to enable TLER. WD permanently disabled TLER on their cheaper drives. This however is only an issue if you want to run raid and then you would have to buy one of there more expensive enterprise drives like a RAID-specific RE4 to have TLER.
This TLER issue however is my only real gripe with WD. Other than that they are actually pretty good drives. I just feel that making it impossible to enable a simple feature so you have to spend more money a slap in the consumers face.
My Samsung drives have been very reliable I have been using 4-2TB Samsung F4's for about 20 months now with no issues what-so-ever.
The issue with Samsung is that Samsung's hard drive division was recently bought by Seagate. I use allot of hard drives and other than one failed WD the rest of my hard drives that have failed have all been Seagate. I have had a very high fail rate with Seagate and will no longer purchase them.
When Seagate bought Maxtor it didn't take too long for Seagate to ruin Maxtor and judging from the most recent Samsung 2TB F4 reviews on Newegg this is already starting to happen to Samsung. To bad they just went on sale for $99 and I was hoping to buy four more before Seagate completely ruins Samsung but it appears I might be to late.
Anyway, with Seagate's recent acquisition of Samsung I would stay away, Western Digital Caviar Black is still a pretty good hard drive for non-raid specific applications.
So IMHO Western Digital Caviar Black is your best choice.