Personnally I try to keep 2 Backups, But one is OK, it's doughtfull both internal data drive and external drive would fail at the same time.
I'd probably go with one or Two of the 750 Gig WD Blacks and the enclosure I linked to.
One other advantage of the 2.5 over 3.5 drives is portability, But I do use Both.
My Backup external drives are never connected and powered on EXCEPT when I do the Backup.
You could go with the Samsung F3 1 TB drive which is a Sata II HDD with good stats . The 1 TB 3.5 In WD SATA III black is also a very good alternative - just stay away from WD green and Blue drives). Need a 3 1/2 inch enclosure (USB3). I also use this combo. I prefer laying the drive down verses installing it upright as less chance of tipping the drive and killing it - I've lost to external 3 1/2 in drives because the accidently got knocked over.