I will keep an eye out for one Samilspec ... and I'll make you a deal on it, if I fine one ... ( better in your hands than mine ..).
20 vs 12 for waterfowling .... I'd stay with a 12 / and balistically you can make a 12ga behave like a 20ga with 7/8oz of shot if you want. Shells for a 12 and 20ga, in my area, cost about the same - so cost of shells isn't a big factor. But Samilspec is right, if you have a good caller and they are working over decoys - you don't need a 12ga / but pass shooting is different and a 20ga isn't going to handle it ( although I agree that a gun chambered in 3" is plenty).
I would use a 20ga for early pheasant season only / or if I was hunting over pointers that were holding the birds close. If I'm hunting over a flushing dog like a Labrador - then I stay with a 12ga and adjust my choke accordingly. And you can make a 12ga perform like a 28ga too - if you can find shells in 3/4 oz - but a 28ga is perfect for me on Quail over good pointers.
It sounds to me like this is one of your first guns - so I would stay with a 12ga for now / add a 20ga later if you want one ... and a 28ga and a .410 ... ( and welcome to our madness ..) - and some reloaders .....