mud,
i own a pair of 12ga (remington 870 and a winchester sx2) and in my opinion thats the jack-of-all-trades as shotguns go. i’ve used mine for ducks, turkey, pheasent, and deer. im nowhere near an expert. but ive asked this question of a number of gunsmiths and it seems that the length of barrel doesnt matter as much as it did ‘back in the day’. with the new choke systems you can get just as tight of a pattern and distance as you would need.
while its true that you will get a little more distance in a longer barrel, you shouldnt notice much of a loss of knockdown power between a 24/26/28 inch barrel.
if you are just getting into shotgun hunting, i would suggest taking a look at the remington 870s. they run anywhere from 300-400 depending on the package you purchase (some combo packs come with a 2nd rifled-slug barrel). and they are, in my opinion, one of the most reliable pump shotguns on the market. they arent anything fancy, but everyone i have talked to has nothing bad to say about them. they are a great starter gun for anyone, no matter how experienced of a shooter you are. and a quality semi auto will run you at least double that. i wouldnt start with one of these as they tend to be heavier than the pumps and they just arent worth the money unless you shoot a ton each season.
but for barrels, i would suggest you shoulder a couple different sizes and see what fits your body the best. a lot of guys have started looking to shorter barrels for ducks because they can turn on birds a tad faster. but i have grown up on the 28″ barrels and prefer the longer length, just becuase im a big boy and feel funny looking at such a close sight, trust me, 2 or 4 inches is a big difference!
whatever you decide on, get a barrel with a removable screw in choke…. this allows you to use the same gun for any type of hunting that you would want.
best of luck in your decision!