Try a good secondhand Italian 12GA (EG Fausti, Bettinsoli, Zoli etc) under and over(U/O) with 5 screw-in chokes[EG skeet 1, skeet2 , half, three-quarters and full ], it is a very versatile outfit for a beginner and all the top skeet shooters use U/Os and at the Olympics, coming up in Sydney OZ.
In the field U/O is also quite OK and most recommended, but I would only favour a pump like a Winchester 1200 for an experienced wild-fowler. Concentrate on field-craft and accuracy/ranges first before blasting away !
If you set up and pattern your loads with the various chokesat your preferred ranges, you can have a very comfortable combination of coping with most senarios and any hunting or sport shooting.
Also a U/O will not boot or hurt you like a lighter gun...AS LONG AS IT FITS YOU. (Very important - also your cheek is the rearsight in instinctive shooting).
May I suggest you borrow different types of club guns and take part in a shoot before choosing ?
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***Big Bunny***