If you plan on running RAID 0, you have one drive that fails you'll basically lose your ALL of your data. Decrease your failure points. The point of RAID 0 is for fast application retrival and writing. As a back up drive you don't want that, you want reliability. You want to know your stuff WILL be safe.
As they aren't answering your question, that is if you're still looking for an answer, 2x 500GB HDD will probably be faster. I say probably because two old 500 GB will be slower than a recent 1 TB, but I'm assuming your buying two recent 500GB hdd in which case RAID 0 will be faster.