I'd suggest you consider buying an external enclosure, such as this:
http://www.amazon.com/dp/B003VKTJGW...UTF8&colid=R75PP4I2A0BE&coliid=I3DOKZ31SP7539
Then, buy a "bare" HDD of your own choosing.
It seems that as of late, HGST (Hitachi) drives seem to get high marks for reliabilty.
The advantages of doing it this way are:
- You "put it together" (all you need is a screwdriver and a couple of mins), so if there's ever a problem, you know how to take it apart for diagnosis
- Upgrading the drive is easy, as well.
I'd suggest 2.5" form factor.
Another path to consider:
For backups, you might consider a USB3/SATA docking station (many, many are available for around $25).
Now, you can get one or more "bare" drives, and swap them around in the dock as required.
Even if you're going to use Time Machine, I would also recommend that you create at least one "bootable cloned backup" using either CarbonCopyCloner or SuperDuper.
If you ever get into an "I can't boot!!!!" moment, having such a backup will prove to be invaluable -- can get you back up and running in about 2 minutes...