Well thats not really much of a poll is it? Which do you mostly use would kind of depend on if you have a CD writer or a DVD writer, obviously you are going to go with the greater capacity.
I somewhat obviously use more DVD's than CD's because of this and I can back up much more information on them LOL
I don't really use CDs any more, as I download all of the music I want to listen to from Napster, and I use my PSP as an MP3 player for them. I still use CDs for some old Windows 98/95 games I have, but I mainly use DVDs now because all of the games I buy are on DVD-ROM, and I can use DVDs to store up to 9gigs on a dual-layer disk and I watch movies on DVDs.
CDs for car stereo, DVDs for movies, CDs for small files, DVDs for large files (and the +/- RW variety), memory stick or external HDD for anything temporary.
If you put an option for that in the poll, I'll vote!
DVD simply because most disc images I find now (shhh, they are pirated) are bigger than a regular cd, and usually only marginally. Borland 2007 disc is 800mb, which is annoying. That and with any good linux distro, the DVD version includes more packages which makes installing a lot easier (when it comes to obscure drivers etc.)