SX3 a fine choice.
I have the SX3 in 20ga walnut field

and A mate has the silver hunter in 20ga as well, (both 28") both have proven very reliable and easy to maintain, 6000 rounds threw the SX3, had a couple jams only one my fault as trying to catch the shell. Have only stripped it for a full clean once,
I persanaly concider both to be amoung the best value gas guns on the market,
They are both made in the same factory as far as I know, and the internal workings seam the same,
Personaly for a unpland gun you might want to look at a U/O or SxS as well, pretty rear to get more than 2 good shots of at upland birds, althought you won't get much of an U/O new for the price of the Sx3.
Upland lots of fun and good excuse for a fine sub gauge, I rearly use a auto upland any more, normaly a 16 or 28ga U/O, aslo if you are group hunting a break action is probley safer, and much easer for others to identifie as being safed.