A CD drive will take up more room than a HD and you can get a cheapo hard drive for the price of a CD drive.
If space is an issue, the HD wins.
If convenience is an issue, HD wins.
If placement is an issue, HD wins.
If performance is an issue, HD wins.
If price is an issue, it's a tie.
There's no reason not to use a hard drive. Keep a back-up on CD if it makes you feel safer, but likelihood of crashing is slim. You don't need an old drive. Get a cheap, slow, new drive. Faster drives wouldn't benefit you on a old mother board's bus speed anyways, and slower drives are cheaper too.
To me, he best place to get a cheap HD is:
http://www.ubid.comIt's an auction site, but not like Ebay. Imagine a Best Buy with everything on the shelves up for auction... (Though it's harder than it used to be to get a deal ever since it got more popular.)
Even better, convince a friend he needs to upgrade his hard drive and then just steal his old one.
