If you're a novice and just getting started, go with a light wieght 20, ammo will be muuuuuuuuuuuuch cheaper! And you'll have a much better selection, especially if you might want to bust a deer or gobbler occassionally. FWIW, no buckshot or slug loads are offered for a 28 gauge.
The big and only real practical reason there are "magnums" in the shotgun world is to put more pellets into the pattern and down range. This however can have a very negeative effect on pattern performance and consistency. Each gauge is optimal with a certain amount of shot and even shot size, but ignore size for a moment. If you consider the shape of a Coke can, its round and cylindrical in shape - the same way shot pellets are loaded in a shotshell. What most shooters don't understand is that the longer that shape gets, or the taller the shot "column" gets because more pellets are added to the "shot column" or "shot charge," the less balanced and more strung out a pattern is going to get. There's alot to grasp at ounce when learning about shotgun physics and dynamics, but less is usually more in shooting a well tuned shotgun, not MORE, MORE, MORE!
A twenty eight gauge does its best with a 3/4 to 7/8 ounce load of shot. The 20 gagger with 7/8 to 1 full ounce of shot, the 16 with 1 to 1 1/8 and the 12 with 1 to 1 1/4 ounces of shot. The smaller the gauge, the better it will pattern with smaller and smaller sizes of shot too. This is because the less amount of shot you have, the pellets need to be smaller for there to be many of them (if you're shooting an once of shot in a 16 gauge and have 300 pellets in that shot charge, for there to be 300 pellets in a .410 the pellets have to be considerable smaller). Conversely, the larger the gauge, the better the patterns will be with larger shot (bird shot is what I've been implying so far, but buck are buck shot when they reach a size larger then BB or BBB. The main thing to remember about pellet size is when it gets bigger and bigger, pellet count gets fewer and fewer. In my opinion, pellet count in a pattern and distribution are far more important overall, than individual pellet size or individual velocity as well.
I've never shot the 28 gauge, but I know is rapidly nearing the end of its relaible perfomance when pellets get larger than 5's for example, and if it was me, I'd try to limit what I'd shoot to 6's and smaller. If I needed something larger than 6's in a pattern, I'd get a bigger gauge or try "fudging" by shooting copper or nickel platted shot instead of just lead. Copper and nickel plated pellets pattern better and have truer "flight characteristics" than lead, and they often cut through feathers and penetrate the game better too.
But understanding and appreciating the finer qualities of the different types of shot is a who nuther can of worms too! Can be extremely important, but not necessary at the moment and more info than you want perhaps.
With what you might be shooting however, #7 1/2, #8, and #8 1/2 size shot might be better choices of shot size than 6's.
To get back to your question, a 28 gauge is a pleasant and sweet gauge to shoot, and it does its job remarkable well when used "properly." Same can be said for the 20 gauge, although the 20 is more versatile and can shoot 3 inch shells stuffed with 1 1/8 or 1 1/4 ounces of shot. Like I said earlier, less is usually more in shotshell perfornce, although the "magums have their uses. I believe most shooters "overuse" magnums however.
Also, almost every American shotshell and gauge out there uses 2 3/4 inch length shells as their standard length for field and target loads. The 28 Ga only comes in 2 3/4 inches as well as the 16. The .410 comes in a 2 1/2 inch length as well as a 3 inch length. With the use of non-lead in waterfowl hunting, the 3 1/2 inch 12 gauge was developed, although 3 inch shells have been available for quite some time in the 20 and 12 gauges.
Europeans shoot some odd gauges like the 14, 24 and 32, but they're rarely seen in the US.
If I was in your situation and really looking for a gun for a small-framed shooter or novice, I'd get a small-framed, light-weight 20 gauge with 3" chambers in an auto. I'd rarely use 3" shells however (that rare occassional gobbler comes to mind, or even #2B buckshot at a whitetail from a tree stand), but the option to use them is built into your choice of firearm.
For quail, doves, grouse and even released pheasants and chukar, your twenty with 7/8 or 1 ounce loads would serve you very well! If you ever got into reloading and were shooting a pumpgun or over/under, you could concoct 3/4 ounce loads (reduced 20 gauge loads) to shoot at skeet at normal ranges and if you decided this was necessary, to use on quail for instance - if they were flushing really close to your gun and 7/8 ounces seems a little heavy. BUT! Reduced loads might not function in a semi-auto gun. in an O/U this wouldn't be a factor and in a pumpgun you're the power behind its function.
As a beginning shooter, I'd recommend you get an automatic instead of an O/U, unless you're already well experienced. A gun like an auto will allow you to shoot slugs and buckshot at deer and might be a better choice for turkeys too.
Hope I haven't overwhelmed you with this post. Surely others will add their resposes too. I've already said too much although I've studied and participated in hunting, shooting, and reloading for over 30 years.