Polishing is taking a piece of aluminum and, err, polishing it. What you see is aluminum with a clear-coat over it.
Chromed is putting a layer of chrome over the aluminum. What you see is the chrome.
Chroming adds weight to the wheels but it is shinier than polished aluminum.
Personally, I like bright silver paint. I had a set of 5Zigen S08 rims that were painted bright silver and they were stunning.
Rims with color (white, black, anthracite, etc) are difficult to do correctly. Some cars work well with them (blue Civic Si with white rims), but it's easy to make a car look very tacky.