All these drives are fast and reliable enough for me, so I focus on my biggest concern: noise.
For a sub-1TB drive set, the Samsung 250 still seems to be the current leader. You can get several of these into a RAID5 or RAID0 set (I don't recommend RAID0, but it is what I am using for now... then again I back up to tape weekly). The noise penalty to run multiple drives is about 3-4 extra dB. In my quiet-pc case setup, this does not raise the noise level to an audible range.
Seagate is occasionally bandied about as a good quiet-pc drive and for most users it's a good choice. But while they have a 500GB drive out now, its 4dB penalty is "pre-paid" as noted in the review tables. Doing a RAID set of 3 Seagate 500's would be louder than a set of 5 Samsung 250's.
Seagate's real disadvantage in noise though is in the frequency range. The Samsung's sound pressure comes at an unusually low frequency. While this makes for a more omnidirectional noise, it also makes it much more likely to blend in with the ambient room noise and less likely to be noticed. Seagate's higher-frequency noise can be aimed away from the user, but will be more noticeable to most people as their ears pass through the PC soundfield's "focal points". These focal points will vary of course depending on the PC case build-up.
For HTPC in the living room, the Samsung in as small a setup as possible would probably be best. For the HTPC librarian who wants 500 DVD's or a hundred hours of HDTV on-disk (2TB), noise shouldn't be a factor since it belongs in the closet or, better yet, in a LAN-based box in the basement.
I'll be interested to see how Samsung's 400GB drive does. Common sense tells me that, with more platters, it will be louder. But it would be nice to hit do 4 of the 400's instead of 5 of the 250's... especially since my A8R32-MVP's main RAID chip has only 4 SATA ports!