Beware the man with one gun !
Your 870 will work fine on Sporting for now. Go pick up a couple more chokes, like
a cylinder, skeet and Imp. Cyl. - and shoot those doubles - it ain't rocket
science ! Pump smoothly and keep your head on the stock !
While perhaps not the "optimum sporting gun", a couple of thousand shells later, you WILL be a better pump-gun shooter, and the experiance will definitely help with your waterfowling. Get some assistance from the local pros to ensure your gun fits you properly and " is shooting where you're looking ". If you ain't looking straight down the rib with a quick, smooth gun mount - then there's probably something amiss. See the recent discussions about "Cast" elsewhere in the forum. Length of pull, drop at heel & comb and pitch are all important considerations too. What works for one guy, may ( and generally won't) work for you. Of the dozens of shooters I know, only two have shotguns that I would consider to fit me ... a lot of the others are close, and as many aren't even in the ball park. The fit (and balance) for YOU is the most important criteria.
Generally, 870's balance well right at the point directly under the extractor slot when the action is closed ( same for 1100's and 11-87's) ahead of that point they seem muzzle light and behind, muzzle haeavy and sluggish. A few ounces of weight in the butt stock ( or mag azine tube) brings them from being muzzle heavy (or muzzle light). By all means, try the other fellows guns once and a while when invited. A year or so from now you will likely KNOW what you want, but in the meantime, shoot LOTS and have FUN. Save your hard-earned for quality ammunition. Ten flats of good 1oz. 8's is a pretty good shooting coach for someone just getting started. Listen to the advice of others, try to follow it, and see if it works for you. Above all, have fun and enjoy ! When your scores at Sporting get up into the 75% range with your 870, you'll be ready to make your own decision on what kind of gun you're going to shoot next.
If you just HAVE TO - the 391 IS a good choice - I quite like the "RL" Parallel
Target version in 28" / Optima Chokes. A good used 682 Beretta Sporting or 325/425/525 Browining Sporting in 30" or 32" would also suffice if you must have an O/U.