FWIW, the best reason *not* to carry the M19 is if you use it for self-defense, it'll be a while before your friendly PD returns it, if at all. I rather doubt they'll treat it with the care a NIB deserves. The SIG will not suffer as much.
IMO, an EDC is a tool. Iffn a minty M19 meets that criteria, buy it. If not, your post pretty well sums up the SIG advantage... and in a life-and-death situation, that's the most important time to have all the advantage one can get. YMMV.

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50 Years of DSOTM

Using the term "rational" in regard to gun purchases is probably a bad start to any discussion. Somebody said "carry what you're comfortable with" or something like that, and that's probably the best advice here. I can't recall a firearms sale or swap that I haven't regretted, so I've quit doing it. And just to throw a bit of fuel on the fire here, why do so many of you assume that a handgun with more rounds is "more effective for self-defense." In about 99% of real-life self-defense situations, the matter is settled in the first few shots anyway, and a Colt SAA would probably do the job, provided you can hit anything with it. It's about how quickly you can put the other guy down, not how many holes you can poke in the surrounding scenery, innocent bystanders or whatever.

You decided long before writing this thread .. you just want someone to back you because you think your not doing the right thing !!

I have several S&W revolvers including a 28-2 with 4 inch barrel and a Pre Model 36 flat latch 38 special. My carry pistol is a Sig P228 with 16 rounds of 9mm JHP.
My P228 9mm Swiss Police Trade in.

That is a matter of opinion but reality being what it is there is no doubt the best DEFENSIVE handgun is the Hi-Cap medium sized/weight semi auto. Accuracy, balance, capacity and reliability of modern Semi- autos are as good as it's ever been...
HOWEVER-

Far too many people own semi autos for carry and rarely shoot them - either they do not make the time for it, do not have facilities or think it is too expensive considering they think they have to go out and dump a 1000 rounds for whatever reason.

As previously stated the chances of ever needing a handgun for self defense is probably only a point or two better than being hit by lightening so it is FAR better to carry what you are comfortable with as well as having the ability to SHOOT IT REGULARLY. I would have far more confidence in the person who shoots a revolver regularly than someone who shoots his 'CC' gun a few (or less) times a year.

There are also those who carry the most compact plastic things in .45 ACP for reasons I do not understand - the absolute WORST gun I have ever shot was a Glock 36 - why would ANYONE consider a SA with only six shot capacity and in a round that was designed for a full size semi auto (IE 1911) and then never want to shoot it because 'it has too much recoil' Heck a typical .357 revolver only carries six shots but is much more capable of better accuracy, control and reliability than say a compact plastic SA in a caliber it should have NEVER been chambered in.

People who say they 'Own guns for defensive purposes only' scare me the most as they are the ones most likely who bought a gun out of irrational fear, shot it a few times and most likely may never shoot it much or ever.

Bottom line? If you own something with the incredible responsibility that owning a gun carries and you own it for defensive purposes take the time to shoot it regularly, learn it's quirks and idiosyncrasies, be able to safely clear it and reload it quickly and above all make sure you NEVER make excuses for not shooting it - but EMBRACE shooting it. OK - off the soapbox now.

Help... I can't see the forest for the trees!!!

For self-defense, use what whatever firearm in which you have the most confidence and with which you have the most proficiency. Many of us are (for lack of a better term) "multi-lingual" when it comes to shooting handguns (or rifles and shotguns, for that matter), and the "best" weapon for self-defense is the one that is immediate to hand, whatever it may be.

If you are good with the Sig and have full confidence in it, it never leaves your body or nightstand, damn the cost of ammo and keep it! If it's just another "tool" and you aren't really in love with it, ditch it and dig into the toolbox for something else. If you want the Smith, "just because" is plenty enough reason to grab it. At this point, the number of rounds and ballistics are irrelevant.

It doesn't sound like you're really in love with the Sig, as you've already seriously considered making it the sacrificial lamb to the Smith & Wesson gods...

LISTEN TO THE FORCE, LUKE!

Most handguns are defensive handguns. There are a few handguns that are hunting guns, like a 6" Redhawk, but the vast majority are designed, built and sold for defensive purposes. That said, the question is whether you intend to carry the gun or not and if so, how.

IMO in today's world a good SA, like your SIG, makes a good CC weapon. 4"- 6" K frames make good carry/sportsman weapons or HD weapons. For me, the preferred carry piece has some wear on it. I don't see you using the NIB K frame in either role. It's going to be a range gun or safe queen. It's going to be too new to risk a scratch or bluing wear.

.357 SIG ammo is expensive and you don't have a lot of it. And you have other CC/HD guns in "common" calibers. Plus you're taken with the beauty of that K frame. So, sell the expensive to feed SIG, buy the K, shoot it, clean it carefully and put the K in the safe. CC /HD your .380, 9 or .45.

Personally I have an old ex-Aussie M10 as a HD handgun. Lots of bluing wear, but still functions and groups just fine. I may replace it with a P226 CPO. The P226 CPO is actually a little loose compared to other P226s, but it's reliable, accurate, and I'm ok with 9 mm.

A 4" GP100 or 6" 686-1 accompanies me on rounds around the property for varmit control - they both take the same speedloader

If I had a CC license. I'd carry either my holster worn SIG P6 or my CZ 75 Compact. Both the SIG and CZ are incredibly reliable- my first criteria- reasonably accurate, and compact. Based on prices I've seen lately, the CZ might win out over the P6.