My son and his wife have SIG P-228 and M-11A-1 guns with G-10 grips. I've handled them and liked the feel. I think in a hot firefight, a high capacity 9mm has a lot going for it. In Iraq, my son several times got in action so hot that he fired most, if not all, of the 4-5 mags that he carried...after emptying a M-4 carbine! He did find 9mm NATO ball ammo to be effective if well placed, but sometimes shot twice before waiting to see if one round would suffice. When your life is in extreme danger, one often does that...
Guns used were M-9 Beretta and Browning Hi-Power. But he likes SIG's now, too, out of the Army and with a choice. And he can now use JHP ammo!
As a gun for average civilian defense and outdoor use, the stainless .357 revolver with its versatility in ammo is probably a better choice for most people. My M-66-3 is my favorite all-round handgun, of all time.
I think $800 is a little high for a M-66. I'd expect to pay more like $550, maybe less, especially if not with a box and tools. But I don't buy guns Online with hyped up prices and no ability to examine them in person before buying. A friend does buy Online, and is disappointed about 40% of the time.
IF this particular M-66 is well fitted and smooth, it's a fine gun, well worth having. But most of the public would pay more for the SIG. There aren't a lot of revolver people left in the "public" realm.
I'm not heavily into SIG 's and may be wrong, but thought the M-11A-1 has a milled slide, while the P-228 slide is stamped. I'd prefer the sturdier milled slide, and I think it's stainless, although black-finished. If I had more disposable income, I'd buy a M-11A-1. It's a nice gun, no doubt very effective in its role. But I wouldn't trade my M-66 S&W to get one.
I do think the SIG ad saying/inferring that a Navy Hornet pilot's other weapon is an M-11A-1 is misleading. I haven't been able to verify that ANY M-11A-1's have been purchased by the Navy. The pilot might have the P-228/M-11, but many Navy pilots seem to still carry Beretta M-9's. So much for truth in advertising! My candid opinion is that SIG should be forced to prove that the M-11A-1 is used by Navy pilots or be charged with false/misleading advertising. I think that if you read the ad very carefully, it doesn't say that specific model is used, but the ad strongly infers that, to promote the M-11A-1. That's just wrong, in my opinion. It 's a good gun, but the marketing is somewhat immoral, I think.
Whether the M-66 or the hi-cap 9mm is the better gun depends on your anticipated needs. But I think the average buyer today will pay more for the SIG .
I think most will agree that you did well. Now, on a sig forum, I dunno. I know I'd prefer the 66 but as mentioned and for me, the value of a genuine NIB popular discontinued model poses a problem in that to retain it's NIB value, it needs to remain unfired. Which is hard for me to do as I'm a collector of shooters, not safe queens. But this is a problem I can live around. 
