Hard Disks are cheaper and better, but it really sucks when one dies. You lose alot of stuff, and you’ve gotta redownload it if you can, which takes alot of time, and can have problems if you have a capped ISP.
I haven’t used bd-r’s yet, but i would assume at some point (if they aren’t already) they’ll be more stable and more affordable.
Alot of people are pro-hard drive, but damn they drives keep getting bigger and bigger, which means when one dies, you lose that much more.
I have a fair amount of hard drive space, but I like to use optical discs to back it up, and archive it. I trust blank media more than I trust a hard drive.