A dedicated skeet gun is going to pattern different than a field gun. It is also going to vary in balance and stock fit. Skeet, Clays, Sporting and Field all can have very unique guns, but you can play them all with a basic mid weight 28 inch barreled O/U with multiple choke tubes.
Here is an older article that explains the terminology a bit.
http://www.herosarms.com/Selecting.htmMy 12 has 28 inch tubes as does my CZ 28 SxS, my 20 Rizzini has 27's fixed chokes and my 28 Parker has 26 fixed chokes.
The 2 fixed choke guns were built specifically for upland and were never intended for the range, but interesting to explain how fit is more important than almost any thing - The Rizzini I had custom made in Italy, I was measured and fit for the gun, although built as a pheasant gun I have shot my best sporting clays round with it over anything else I've ever shot, however it did kick the crap out of me doing it.
The moral is all this info is great and it will make your head spin but in the end get what you can shoot well, it may have 26 or 34 inch barrels, it might weight 8+ lbs or be under 6, the length of pull or rise might not be within the norm, but if you can break what what your pointing at thats what counts.
Much like handguns I'd strongly encourage you to try before you buy, $2000-$5000+ mistakes are a lot harder to live with than $500ish on a handgun you don't like and the shotgun is much harder to re-home.