It really doesn't matter what media you select. The important part is your schedule of updating. Ex.. You pick optical discs and you burn a new disc every 3 months and store it securely. The important fact is not the storage media, it's the 3 month schedule of updating it. You can switch media out on your 3 month update schedule, use flash media, external hard drive, tape, whatever. As long as your most recent backup is no more than 3 months old you'll still be able to read it.
Where most people fail at this is the schedule.. And that's why an online backup solution would work better for the majority of people. Schedule your online backups with one of the secure vendors and let it run automatically.