Okay, first and formost, get the Ruger 10/22. View it as an educational and recreational gun rather than a home defense gun. Lightly used figure to spend $200. New figure $250.
For a self defense gun, especially on a budget, go to your local gun store and walk over to the USED SHOTGUN section and pick yourself out a lightly used pump action 12 gauge. Again, budget about $200. If you can get one with an 18 to 22 inch barrel that is best. DO NOT GET ONE WITH A PISTOL GRIP ONLY!
Third, remember, the lighter the gun the more it will recoil. The Ruger SP101 is a small gun, but built solid. It is going to be the softest kicking of the small revolvers, but still, I suggest if you go the way of the revolver to look at a medium framed gun, like the Ruger GP100 or the Smith and Wesson 686.
Note #1, if you are planning on carrying the gun out and about, you will most likely need a concealed carry permit.
Note #2, if it is a 'house gun' then the small size is much less relevant, so that makes me think a medium framed 4 inch revolver or your suggested Springfield XD would be a good choice. If, however, you do get a concealed carry license, it has been my experience that people generally don't like to carry heavy guns, and though the SP101 is lighter than a medium frame it is the heaviest light frame. For 'shoot rarely, carry a lot' I suggest a hammer shrouded pistol with a lightweight frame like the Smith and Wesson Titanium Bodyguard.
Now, with Obama in office we are likely to see another AWB or similar silly laws. If you are thinking about getting a Springfield XD, I suggest getting one NOW and getting a couple spare mags in addition to what is included with the gun. Combine this with the above statement of 'size of a housegun doesn't matter' and the 'you need a CCL to carry' I strongly suggest purchase #3 be the Springfield XD,
and only when you actually have your concealed carry license should you make purchase #4 of a snubby revolver.
As far as keeping the wife's fear of handguns, two things to do. Educate her and buy a Gunvault and bolt it to your nightstand.
http://www.sportco.com/productcart/pc/viewContent.asp?idpage=28&gclid=CK_Q27aV0ZgCFQ6jagodEgQa1A
Then stick a light on the Springfield's rails and stick it the box.
final note. You may want to consider buying a handgun that your wife would be comfortable shooting. Many women end up being able to handle the biggest heaviest recoiling guns around, however, be aware of hand size. Most higher capacity handguns are built around a hand that would fit in a Mens size large gloves. I've known some women with hands that big (typically farm wives who could outwork a half dozen white collar cubicle drones) but that is not the norm. Small hands of any gender tend to work well with 'single stack' semiautos. This means 8 or so shot capacity rather than 15. I think this is why a lot of women end up liking to shoot 1911A1 'colt 45s' even though some guys balk at the recoil of a 45acp. Anways, I am rambling, I suggest considering a 'his and hers' rather than the springfield, and the Walther PPS is a wonderful gun because of how slim it is. Note. PPS, and NOT PPK. This is NOT James Bond's gun. I think this handgun would be an excellent fit for teaching young sons and daughters as well, who also often have small hands. Heck many guys who spend time typing on keyboards rather than using a lugwrench would probably find the Walther PPS a superior fit to the wide Springfield XD too.
Of course, don't just assume your wife will like ANY gun you buy, make sure she has her input too on a 'his and hers' and for a 'her only' leave the final decision up to her 100%