Readymade External Hard Drive vs. HDD + External Enclosure?

What are the pros and cons between, say, the WD My Book 3.0 and the Samsung Spinpoint F3 in conjunction with a USB 3.0 enclosure such as this? I'm in the market for a new external HDD and a lot of what I've read seems to suggests that buying the hard drive and enclosure separately is the way to go, but I don't fully understand the reasoning. It doesn't seemed to be based on cost, so is it performance?

Also, if I decide to go the F3 + enclosure route, would I be better off getting the USB 3.0 enclosure I linked to above, or an eSATA one like this? Either way I'll have to get a corresponding expresscard for it to work with my laptop (a Dell M1330). If you have any better suggestions for HDDs or enclosures please share them, but keep in mind I'm going to be buying from Amazon since I have a gift card.

 

Really, the only reason to buy a separate USB enclosure is if you already have a spare hard drive lying around, and need to find a place for it. Otherwise, just get the pre-assembled hard drive enclosure.

Just be sure to buy from brands that are known to be reliable, and have a good warranty. Western Digital, Samsung, and Vantec Nexstar are all reliable brands, so you should be ok there in case anything goes wrong. Beyond that, the only real choices to make are size, capacity, and connectivity (USB, eSATA, etc). You can actually get by just fine on a Dell XPS M1330 with just basic USB 2.0... it transfers at up to 30MBps, which I find to be sufficient for any kind of archive transfers.

If you want full-speed transfers, then you would need to go with eSATA or USB 3.0. I generally prefer USB, since it does not require additional power for 2.5" drives. However, since you're using 3.5" drives and will require external power no matter what, I would go with eSATA for the lower CPU overhead.