Very well said Eiger5. I have three pistols that I like for carry; all 9mm; Beretta PX4 Storm compact, XD Mod.2 3". and new XD-E. All are very good shooters and each has its advantages. Accuracy is excellent for CC or anything else. Actually, I prefer the DA/SA safety/decocker with the Storm and the XD-E. Extra level of safety and flexibility. I even sometimes shoot 3 gun with the Beretta and the Mod.2. Otherwise a Glock 19 gen 4.Some of us here would not see a 9mm as a compromise, but rather as our first choice. It is a great self defense caliber with higher magazine capacity, you and your girlfriend will be able to afford to shoot it more often (have you compared ammo costs of 9mm vs. .45?), which will make you more proficient, and you'll be able to enjoy range time together.
I have nothing against .45s (carried one for 20 years), 10mm are cool, I have a .40 that I really like, but if I can only have one gun, it'll be a 9mm. IMHO, they just bring more versatility to the table.