I think both DVD-ROM and CD-ROM are the most suitable media for backups, just make tow copies, and save it in well closed closet away from dust and sun, then they live almost 50 to 100 years.
in case of cheap there is DVD is best but if you wanna a back up for the large amount of data there is need of the Hard Disk drive.
Files system will be more suitable because their is can be error or problem in other storage for data backup.
Backing up online is your best bet. Some programs offer local backup as well. Check out Mozy, they even have smartphone apps.
Backing up to only 1 place isn't best. I have all my important stuff (source code, passwords [secure, of course]) on SD cards, an external drive and Dropbox. It's not being paranoid. If I lose my stuff I'm out of business. And the grocery store won't give me food for free. Storage is cheap - if you need the data, make sure you can recover it.
I would have to say all of the above plus online backup. Comes down to how valuable your data is because most of the time you can't put a value to your documents. So might as well take precautions then running IF scenarios later. I backup my most important stuff in few different hard drives, online, pen drive.