I like both - I agree polyester is more "lofty", but sometimes that's not what you want!
I also often find a need to use black batting, and so far I haven't seen cotton batting in black so I use black poly. I've never tried Hobbs (which I know has a blend in black) because I tend to need to quilt farther apart than it allows.
If I want shrinkage and crinkles - cotton. If I want puffy and smooth - poly!