I carried a sidearm for 23 years and trained others for 20 years. Duty holsters, drop holsters, outside the waistband concealed and inside the waistband concealed. Comfort is very important otherwise you will not want to wear it much. Therefore size, weight and proper holster are big considerations. However, here is the biggest... can you quickly draw and hit your target with it under the most stress you will probably ever experience. ( a lethal encounter) That is the real reason people CCW isn't it? Forget about looks, it's not a fashion show.
Look for pistols in the size/weight category you are comfortable carrying and then try to shoot all of them. Many ranges rent pistols, especially in the U.S. You need to be able to make quick, solid hits from the concealed draw, often with one hand. I read a study out of the states where out of approximately 250 close-range pistol fights, only 7 people were able to get both hands on the pistol before firing. Your support hand might be busy defending from the attacker, pushing someone else out of the way, trapped against your body or you might have just enough time to clear the holster and level the pistol before you fire. Too bad this type of training is hard to find for civilians.