Cowdogpete - I have no experience with the 10ga, but I am a fan of 16ga shotguns. In fact, over the past year, I've wound up with an old A-5 and an even older Model 12 in 16ga.
The more that I've shot, the more I realize that anything over an ounce of shot isn't necessary (as long as you're shooting lead). While there are exceptions (such as wild pheasants a few weeks after the opener), most of the upland birds can be readily dropped with 7/8 or 1 oz loads.
A 20ga can push that amount nicely. However, a 16ga should do it a little better. A 12ga would probably be better still, but the gun itself is going to be a little heavier, which by the end of the day can take its toll.
When a 16ga is scaled correctly, it's noticeably lighter than it's 12ga counterpart, making it easier to carry. Especially when you are chasing an energetic lab, which is in-turn chasing a fleet-footed winged critter that refuses to take to the air.
The downsides of the 16ga are primarily:
1) Lack of availability of ammo. Even some of the well-stocked places in town only have a few lonely boxes of 16ga on the shelves. It does seem that this has gotten better over the last couple years, though. Try to find some at a gas/ammo/bait shop in the middle of nowhere, and you're probably out of luck. 12ga and 20ga is usually abundant.
2) Lack of variety of ammo. Related to number 1 above. Since the shops don't carry very much, they usually don't have much variety. Sometimes, all I've seen are 7 1/2s or 8s. Ideally, you'd want to have 5s, 6s, and 7 1/2s for game. Also, I've yet to see any target loads on the shelves. Forget about a ValuePack at Wal-Mart.
3) If you do as I do (which I don't recommend), I often have a whole host of shells rattling around in my "huntin' bag". Fortunately, all of my 16ga shells are blue, so I can keep them seperate. However, I've personally seen a friend jam a 16ga shell in the barrel of a 12ga pump. Fortunately, he caught his error before he fired a 12ga shell behind it... and kept his face intact. We did have to cut a long straight branch and pound the thing out of the barrel though.
4) Older (like 1930's-vintage and before) 16ga guns often have short chambers... usually 2 9/16". These can be unsafe with modern loads, and in any event won't cycle properly. In many instances, the guns can be converted (by lengthening the chamber and recutting the ejection port at a minimum), but such conversions often run a couple hundred dollars. If you don't go that route, then there are places to order 2 1/2" shells. This is what I've done, and I've had very good success with those shells. I'm saving the hulls, as you can fit a MEC reloader with an adapter plate to load the shorter shells.
For me, I just kind of like quirky stuff (which is why I've got a non-functional BSA motorcycle in the garage) and the 16ga fits that bill, without being impractical. In fact, since most of my hunting is for upland game, the 16ga is just about ideal. I plan to do most of my hunting this year with the 16ga Model 12, using either 1 oz of #6 shot or 7/8 oz of #7 (not #7 1/2). If all goes well, I'll be shooting dove, grouse, pheasant and maybe woodcock. By the end of the season, I should have a very good idea of how effective the 16ga is. In the past, I've almost always used a 12ga, unless I know in advance that we'll be out all day in pretty nasty terrain/cover. Then I'll sometimes bring a 20ga instead. This year, I'll split the difference.
SX2 is a great gun - no doubt.
