I'm know I'm comfortable with the MP platform, but I don't have experience with the 1911 style. Bottom line, the Sig is a better gun with regards to quality and value and is where I'm leaning.
If your wife is going to shoot it, be sure to include her in on the decision. As the gun isn't new, go out and shoot the sig first to see how you like it. You may not like it. If possible, I'd recommend going to a range that rents pistols and rent a few. It will cost some money, but will be worth it so you don't buy something you end up disliking. This isn't a recommendation or a warning, just my personal preference: I don't care for the 1911 platform (although the removal of the grip safety takes away one of my gripes) especially for a new shooter, and I don't care for the 380 round (I'd rather have a 32, but prefer 9mm).
For home defense, if you don't want a long gun, I'd go with a full sized or compact pistol, like the M&P 9 or 9c. Personally, I don't like the trigger on the M&Ps, but I am a trigger snob. The best trigger on a striker fired pistol is the Walther PPQ, which also has similar ergonomics as the M&P, but is $100 more.
It's the golden age of handguns in the US. The choices are endless, whether it be a used S&W wheel gun, to a Swiss copy of a CZ, to a Korean take on a S&W auto, there is a handgun for everyone. And prices are quite low--M&P 9 on sale for $386 shipped at grabagun.