Use the same gun if possible
If you and your wife are going to have carry permits, I would advise that you go for a mid size 9m.m. semi-auto pistol like the GLOCK 19, WALTHER P-99 or if you prefer a metal gun, then the SIG 228 or 229. I have had good results with all of these guns as well as the BERETTA 92 Compact which is no longer in production.
If you want something smaller that can still fulfill the home defense role, I also like the SPRINGFIELD ARMORY XD Sub-Compact. For concealed carry, you can use the 13 round mags with the larger 16 round mags for home defense. I have found that PEARCE EXTENDERS make the smaller mags easier to use for average size males.
While the revolver is often recommended for its simplicity, you should consider the negative trade-offs.
You will be getting a weapon with a MUCH SMALLER AMMO CAPACITY.
If you are separated from you shotgun, you also may not have a RELOAD handy. I COLT PYTHON that has fired all six rounds is just an unloaded gun.
I recommend the 9m.m. over the .40 S&W or .45ACP because of the lower recoil. This is especially important as the gun gets smaller.
Besides, you already have a shotgun if you need more power.
A mid size like the GLOCK 19, P-99 or 228 is small enough to be carried concealed with a still adequate ammo capacity and good control. Recoil is the biggest problem with compact handguns.
I would recommend that you go to a range and rent these guns or something of similar size to see if they work for you.
Also, remember that your wife may not like the same gun as you depending on her physical size and interest or experience with handguns. Don't put her off be trying to decide for her. Also, avoid treating her like a weaking.
After trying about a dozen guns for feel, my ex-wife and I went to the range and the gun she liked most was the most powerful, a SIG-225.
The grip was small enough for her to get a good hold, while the trigger, recoil and sights were just right.
She had formally shot a GLOCK 19, but as she got older, the recoil of the 19 (which is about half a pound lighter than the SIG 225 became a problem). The 225 is just right as it is compact enough to conceal if needed and a faster reload and high magazine capacity than a 5 shot .38 with much better control and a more powerful found.
Hope this helps,
Jim