First off...from a TCO persective and protection and stopping power; you want a .40 cal. Anything less is not going to stop anything and you're just throwing lead at it. I have an M&P S&W .40 Cal and a Beretta Px4 Storm Sub. I also have a 1911 Kimber .45 Cust Covert. I would recommend trying and buying a .40 cal.
You also want to pay attention to double action and single action and or striker action. I also recommend a gun with a hammer. Which would elimate most glocks(or all), which i don't personally prefer.