If it's skeet, and ONLY skeet, the 20 is all you need. Now for the arguements pro and con....
If you buy the 20 and later decide you'd like to shoot trap or sporting clays, you're at a disadvantage with the 20. This is, of course, a perfectly valid reason to go out and buy another shotgun.
If you buy the 12, you can then shoot trap or sporting clays and not be handicapped by the smaller gun. For skeet, you can get 7/8 oz loads - but you'll probably have to order them by the case, 'cause nobody ever seems to carry them in stock. This will make you decide to buy a reloader... see how it works?
If you buy the 20 and later decide you'd like to shoot trap or sporting clays, you're at a disadvantage with the 20. This is, of course, a perfectly valid reason to go out and buy another shotgun.
If you buy the 12, you can then shoot trap or sporting clays and not be handicapped by the smaller gun. For skeet, you can get 7/8 oz loads - but you'll probably have to order them by the case, 'cause nobody ever seems to carry them in stock. This will make you decide to buy a reloader... see how it works?