A sponge will trap the dirt and grit, and wipe it across your paintwork, causing swirls and minor scratches. A mitt will help lift these particles away from the paint, minimizing the chances of this. Coupled with some decent products and a good 2 bucket method it'll keep your paint in tip top condition for much longer. Also drying your car properly is very important, no chamois, and no blades. Have a read through some of the guides on here, you'll be a certified detailing nut in no time.