The major differences are, like you said, spped and capacity. DVDs spin 3 times faster than CDs and contain approximately 7 times as much data.
Physically speaking, they have the same diameter and thickness, but DVDs consist of 2 layers at .6mm each, while CDs have a single 12mm layer.
As far as life span goes, check this out. (From PC Magazine)
CD-R: 2 - 15 years
CD-RW: 25 - 30 years
DVD+/-R: 5 - 15 years
DVD+/-RW: 25 - 30 years
Hard drive: 3 - 6 years
Flash drive: 10 years
The range is due to the different brands available. Memorex and Sony would fall at the "high end," while Officemax brand would probably fall at the "low end."
Given this, it is best to back up your data on a +/-RW disk (either CD or DVD) from a good company. You'll pay more for it, but it's worth it for archival purposes. Just make sure that you DO NOT use a solvent-based marker or CD/DVD labels on the disks themselves...they will slowly eat through the disk, causing permenant damage. (It's best to leave the disk alone and put it in a labeled jewel case.)
Finally, I have heard of "archival DVDs", just like the special photo paper that's supposed to last 100+ years, but haven't seen them available. (May only be available by special order.) The government is currently debating whether there should be a "standard" for backing up the country's data. We'll see where that goes after HD-DVD and Blueray come out. 
Hope this helps,
John