My iPad 1 was 32Gb and I never came close to filling it up.
So I will get the 16Gb this time around.
I don't keep any music on my iPad and very few movies and when I do put movies on it I delete them when I'm finished watching them.
I second that. My iPad 2 was 16GB and boderline full partially with music, but now I've bought a Lumia 800 and will have no need to put music on my iPad 3. Just like you I delete Dvix movies after having watched them and with regard to apps I'm pretty modest and only keep about 10 apps on my device which I actually use on a daily basis.
So 16GB will do the job well. Lets just hope that too many app sizes don't creep up to 1-2GB a a pop.