I would always use tape over ANY type of optical media... My job is server/backup administrator for a 4,000 user company, and I am constantly asked to restore things from tape by people that were silly enough to use optical media as an archive/backup medium.
You can pick up a cheap DDS3 drive for $20 off eBay, and get an cheap SCSI card to suit for about the same $$$
At my work we have tapes that are years and years old and still work fine, however you cant say the same for CDRs or DVDR's
If you wanted to get something a bt faster you could go for an old DLT7000/8000 drive which will give you 70/80GB compressed and up to 10MB/second transfer speed.
Something like :-
Would work perfectly. DDS Tapes will cost under a dollar each and hold 12GB native and 24GB compressed. For well under $100 (for the DDS) or a couple of hundred for DLT you effectively have an enterprise class backup solution.
It all comes down to how much money your data is worth to you.
Hope this helps,