There is only one major rule when it comes to carrying a pistol: the one that will save your life is the one you are actually willing to carry. This means that if you buy a weapon and don't carry it because of its grip, its weight, capacity, calibre, stopping ability, whateverthe**** - it won't do you any good.
I know it sounds elementary but it's amazing how many guns get bought and tucked away into sock drawers because people got too concerned about all the other (admittedly, interesting) bull**** when buying it, and not concerned enough about what the thing feels like when sitting in a passenger seat for 16 hours on a surveillance, or how it feels on the hip while working a door all night (or more importantly, whether or not it's got an aftermarket locking holster made for it, which I suspect is VERY important in your situation).
Don't buy a .40 just because it puts out greater 'stopping power' numbers in ballistics gel tests if it feels less comfortable than that 9 or .380 super of your friend's you might have liked. And don't get a seven-shot .357 for the sake of having the extra round if you think it may be more bulky, and a pain-in-the-ass to carry, than that six-shot you enjoyed trying out at the range.
Again, I know this sounds elementary, but it's a lesson that's usually hard-learned. I have a friend who saved his own life carrying a two-shot .22 Derringer. My 150 pound father, on the other hand, was near-killed and left with a broken jaw, a full set of dentures, a broken hand, numberous broken ribs, and a full metal-reconstructed face (complete with titanium screws) after being blind-sided rolled and left to brawl in Mid-City New Orleans with a couple of ex-Angoa, ahem, brothers. Why was he unarmed? He had gotten tired of carrying the only pistol he owned at the time, a .32 revolver, because he couldn't find a good holster for it.
A 'perfect balance of stopping power and magazine capacity' did ****-all for him.
Get the gun you are willing to carry and can work hours on end with.