For long-term bulk archival storage, you can't beat a Hard Disk.
DVD's are pretty crap for long-term archival storage
CD's can be okay depending on the dye - Cyanine discs (Deeper purple/blue) are rubbish; 5 years and it'll be totally unreadable.
The best is usually PhthaloCyanine (Light green/gold) - These often have higher error rates and don't burn as cleanly, but the dye is much more stable and is rated at 100 years (BS, but the real figure is likely still much higher than Cyanine).
DVDs degrade much nastier than CD's, presumably because of the track sizes.
You can keep them going for quite a while if you put them in a cool dry dark place 'tho. Warmth is the most dangerous thing to CD/DVD media as it accelerates the breakdown of the dye.
Storing a CD/DVD in direct sunlight will probably kill it pretty quick.
Hard disks OTOH last for ages as long as you don't do anything stupid. If you only plug it in occasionally then it will probably out-live you!

And you can pick up a server-grade 300GB ATA hard disk for like, £50 these days!
(Just don't look at SCSI... a 300GB SCSI hard disk would be more like £600

)
The ultimate long term storage would be some sort of MO (Magneto Optical) drive, but good luck finding one with any decent capacity these days!
Oh, and re. the thing about burning CDs slower - This is actually false, and can actually make the disc worse depending on the CD burner!
A lot of burners have an optimum speed - Usually 32-40x - If you burn above or below this, discs can actually have MORE errors!
On well made drives, you don't get such a difference 'tho, and in such cases slower speeds can give lower errors.