First of all, it is what fits you, and what fits your shooting style, on the intended game. There is no single perfect fixed-choke SxS or over/under shotgun. If you are not an accomplished SxS shot, you should take lessons from a master, and attend some of the SxS shoots in New York and North Carolina to see a lot of guns and get to try a lot of guns.
None of them are investments, unless they are very high grade, in excellent conditions, and you don't hunt with them.
All of them should hold their value against inflation, if you take care of them.
Connecticut RBL
* Fabulous gun. Try to buy it used, which will probably still be in 98% condition.
Beretta 471 Silverhawk
* The 470 or 471 is a really nice shotgun.
The 12 gauge balances better than the 20 gauge for me.
Don't own one, but probably will before the year is out.
CZ Bobwhite or Ringneck
* Not in the class of the CV or 470, but a terrific value, just like the Ruger O/U Red Labels.
Winchester Model 21
* I own one 12 gauge, really nice, that I bought for $2,500 a few years ago. You need to shop a lot, learn the value of the different grades, so you can recognize a bargain when you see it, because it won't be there very long.
Winchester Model 23
* Some are very nice, but the prices have gotten out of hand.
For that money, you can buy a used Aya, Garbi, or Merkel, which are a lot more shotgun.
Browning BSS
* Very strong, businesslike, yeoman workhorse.
20s go for a premium.
SKB
* Some look nicer than others. There are a lot of grades, and SKB owners trade up, so find an SKB dealer and tell him what you want in a used one.
Parker
* Like my Win 21, I have one, for $2,500. You need to shop a lot and learn the values of the models and grades.
L C Smith
* Nice, but the Marlins built in the early 1970s were just as nice. Buy one of those and save a ton of money.
Fox
* Grew up shooting Fox 16s and 20s on quail. Wide variation of grades, quality, and how shot out they are. In 2005, I sold a restored AH in 16 gauge that was nicer than anything I have ever seen, live or in a magazine, for $1,500. Should have kept it.
Lefever
* I shoot a 12 gauge Lefever sidelock on quail. Before the lead shot ban, this and a 32-inch Ithaca 3-inch F/F magnum were my waterfowl guns. Now I shoot 2.5 inch Westley Richards in it mostly over tightly held quail in January and February. When they get up further and faster in November, I shot a 28-gauge over under Browning 26 in SK/SK.
The modern side by sides are very good, and with chrome lined bores, back boring and screw in chokes, they will shoot steel, bismuth and tungsten shot. Many of them have much thicker barrels than the old Lefevers and Foxes.
Look for an AyA or Arrieta, or Garbi.
The Ugartechea in a box lock is a bargain.
Uga made the Parker Hale, so look for one of those used.
I have also owned a pair of AyA 30s. Great guns.
Also shoot a Churchill 25. Superb gun.
This brings up the bargain English guns, like the Cogswell & Harrison, Webley & Scott, etc. If you are thinking of mostly shooting upland game, where you don't need a big load anyway, the 2.5 inch guns can be had at a deep discount. Still, $2,500 for a cheap English shotgun, would still buy a nicer and much newer AyA.
http://www.doublegunshop.com/aya1.htmYou have to know what fits you before you go buying some older gun sight unseen. But if you do know, there is a lot out there, like the higher end Charles Daly's, older J.P. Sauer, Darne, etc.
There are a lot of overpriced used guns out there, but you should be able to find a really nice piece for $2,500.00.
It also depends on where you hunt, how cold it is, the coats and gloves you will be wearing. For example, late season, early morning pheasant or quail, or ducks, temperature in the 20s, I am going to be wearing a canvas coat over a sweater, and shooting gloves. I want a slimmer grip and double triggers, slick butt, shorter length of pull.