I've used both and find that the sea sponge holds alot more water than the mitt and is alot faster to wash a car than the mitt. Although it uses alot more water, it is very easy to work with and keeps the paint very wet and lubricated. Like FMINUS pointed out, it does great with fine dust and loose particles, but does not clean very dirty cars as well as the mitt.
I use the sea sponge when I use a shampoo and use the mitt when I use ONR.