If the existing shotguns fit you well, are equipped with appropriate chokes for Upland hunting - then why not, particularly since you may already have become adept at handling those firearms. Having said that, you may perhaps have to contend with the weight and bulk of such shotguns as you move around.
A few relevant facts to consider are that Upland hunting generally involves greater mobility and so the ratio of actual shooting activity to walking is usually much lower. In this aspect, a light(er), at least a reasonably well fitting, well balanced shotgun, properly choked (preferably screw-in tubes or fixed) is always a great assist. Will offer enhanced ease of carry and handling in the field.
Breechloading shotguns i.e. SxS or O/U can offer many advantages particularly in the case of upland hunting. However, the choice of certain action types (whether break action, semi, pump etc) is best left to the individual depending on his/her familiarity and comfort using a given type of firearm.
The selection of a specific gauge for Upland hunting may also be influenced by one's experience, hunting scenarios encountered, game species being pursued, use of bird dog etc. You may perhaps wish to look into a 20ga if you do not own any at this time. The 12, however, will be the most versatile in view that you will likely be hunting over more open terrain.
Lastly, the thought of acquiring an additional firearm is always exciting and most of us look forward to such recurrences with great eagerness!
Man, you read my mind!! I was writing this (below) while you were writing that (above)!!
I've been pondering selling off a few rifles in order to buy a dedicated sharptail/hun shotgun. (check the ee, hint hint nudge nudge)
I currently have 3 shotguns, all 12s. An ithaca 37 waterfowl, a5 light 12 and a b2000.
Is it really worth investing in another shotgun or better to use the ones I already Own?
It depends on whether or not you have funds for a new gun, and how much hunting you have planned.
Your collection could certainly use one or more double guns. Having said that, you definitely could hunt with what you have already. In my experience, sharptail are wiley and tough. In the early season, they're found in pairs on the edge of the woods bordering on large open areas. Later in the season, they're found in large coveys with sentrys posted to guard the group against surprises. Throughout the season, you're looking at medium to long shots. I don't see why your Ithica wouldn't make an adequate sharptail gun.
Huns, in my mind, are typically closer shots, well suited to smaller shot sizes and more open chokes. Depending on the choke on your A5 light, it could be applied to this purpose. I'd be looking for something in the Modified to Improved Modified range. Obviously, anything tighter could be opened up a bit.
One gun, well suited to hunting both sharptail grouse AND hungarian partridge would likely have to be a mid-weight field model with removable chokes. Something like a 28" Citori U/O with a full set of chokes.
But, as many have alluded to, and some have plainly said, a nice, well-balanced sxs would be best of all. If you have it in your budget, treat yourself to a nice sidelock or a classic Birmingham boxlock. If you look into it, you can find volumes of information describing the quality, fit and finish considerations that go into selecting the right gun for you. Most of all, don't rush the journey. Years from now, you will be glad you savoured the moment. Pardon me for being trite, but it's akin to your first ###ual experience. Take your time. Enjoy the feeling of having the cash in your pocket and let the supply of guns do it's best to seduce you into parting with some. But, be picky. Ignore the brand names. Look only at the individual in front of you. Touch and fondle (Gently, please! Always prove gun when handed to you, but treat as loaded anyway. Never sweep people with the barrels!). Take the guns apart. Look down the pipe. Re-assemble them. Throw them to your shoulder with your eyes closed. Open your eyes and see where the gun is pointing. Open and close the action a few times. Estimate the weight of the gun. Observe the choke constrictions, numbers of triggers, extractors or ejectors, length of pull and drop, and so on.
Little details matter. Educate yourself before taking the plunge. As an example, I offer you receiver design. Receiver design can make a huge difference in the field. A round action, for example, greatly reduces fatigue to your hands over hours of carrying in the field. Hunting ruffed grouse and/or hungarian partridge lends itself to sub-gauges, such as 20 or 28 ga. Properly made sub-gauge guns are built on scaled frames - lighter to carry, as is the ammunition. Low pressure loads are ideal in these guns, and using them opens you up to a host of affordable classic guns.
A shartail gun is synonymous with a pheasant gun where I hunt. Both are tough birds. I prefer double-trigger guns in the field because I like having instantaneous choke selection. I tend to hunt these species with slightly heavier guns. 12 ga. doubles or pumps are appropriate. Larger shot sizes, like #4 and #5 are a good idea, but don't get carried away with heavy loads - they beat up the shooter and the gun more than they do the birds.
I wouldn't be in a rush to have something in time for hunting season this year (unless the cash will be gone otherwise). Make your first double gun a good experience and you'll be hooked for life.