the expensive one which holds all your data would spin less, get searched less and written on less and therefore be likely to last a lot longer, you can also keep an image of your c drive on it in order to be able to restore to another drive when it conks out.
notice carefully that some items only begin spinning when you hit "my computer"
as i mentioned, i got a 2tb for £70 only a couple of months back, the impact of the floods should hopefully wear off soon,
in any case, if it drags on - china is quick at spotting business opportunities and will mass release them.
but if you're bent on getting it, i would personally buy an external and take out the drive:
then i'd have a hdd for much cheaper, and have a free caddy