This would depend on the tools at hand. If you have the proper chisels and jigs using a blank would be fairly easy, and you would not have to worry about squaring and plumbing the frame.
If you only have a 4' level, a hammer, and some shims hanging a pre-hung door would probably be your best bet.
A third option is if you could find a pre-hung where the hinges would line up with the existing frame, you could simply hang the pre-hung door in the old frame. This option eliminates almost all the work.
In my opinion, if the existing frame is true I would rather use a blank. Most pre-hung doors come with really cheap crappy frames (unless you pay extra for a good one), so any time I've used one I ended up building my own frame anyway.