Availability of nails varies from place to place, but now that the box stores sell so many brands it is not much of a problem finding whatever you need. Go to whatever store you are going to be buying from and see what they have in stock. There are advantages to each type but for homeowners use it won't really matter. 28 degree are more compact and usually paper colated. Less reloading and generally a more compact gun. 21 are usually plastic colated nails which are spaced farther apart so less nails in a clip and some people don't like the little plastic pieces shooting out and littering the site. Some guns will shoot a larger diameter nail needed sometimes for shear strength applications and those are usually 21 degree. You can buy a nail gun in any pawn shop in the city probably cheaper than harbor freight if you want to go that route as well.