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The twenty guage is good advice for several reasons. There is a bit more punch and a several more pellets. If possible get one with a modified choke as you can still shoot slugs, or steel shot and it's better for Ducks with heavier shot.
The 30-30 is OK, but is there a reason you're looking for, I presume a lever action rifle??? Such as being a south paw or left eye dominance??
You might want to look for a Savage Model 340, chambered for the 30-30. It's a bolt action, very strong, so you can handload it hot with pointed bullets and easily mount a scope in the future. It's also about half the price of a Model 94 or a Marlin.