Seven inch tablets are better at two thumbed typing in portrait orientation, while ten inch tablets are better at two handed typing in landscape on a flat surface.
Seven inch tablets are small enough to fit in purses and large pockets.
Seven inch tablets are holdable in one hand for extended periods.
If you want to use your tablet like a computer, get a 10 inch. If you want to use it more like a smartphone, get a 7 inch.
Listening to music on the go on any tablet makes you look silly. Thankfully people don't care enough to comment.
I've based the above generalizations solely on my own iPad 3 and Nexus 7.