As one who was into making movies in college (video now) I can tell you that I used both. Just as I am cross-platform in my computers, so I am with my tape.

Gaffa is great for many things as stated in earlier posts, but Duct is great where you need quite a bit on the cheap. I especially find it useful for... well... central heating ducts.

However, my favorite tape is that stuff that is like thin aluminum. When I want to direct light in a particular direction, I use it to make a snoot or quick top hat.

Great poll. :D Duct tape is the first thing I pack before heading to a gig, it's been used for everything from covering cable on the ground to patching a busted bass drum skin.

I swear by this stuff, it's saved my rump on many occasions.

Another score for Duct Tape
 

Heres another site for duct tape! Pretty cool stuff! :D

I think this is the oldest poll I've ever seen resurrected... Even though I didn't know the name of it, we used Gaffa on a production I did in high school, it's good tape for things like that.

Of course, anyone who's seen the Red Green Show knows that duct tape is the "handymans secret weapon", and can be used to make just about anything...