power, concealability, controlability - YOU ONLY GET TO PICK 2.
Its a trade off. S&W makes a scandium .44 magnum with a 3 inch bbl. I'd say thats the ULTIMATE in power and concealability. But it will take you 2 seconds between shots to recover from the recoil, muzzle flash, cylinder gap flash. (remember 90+% of defensive shootings happen at night)
Bottom line, don't forget about control. I'll give you an example. Years ago I wanted to buy a subcompact Glock. I looked at the 26 and 27 in 9mm and .40 respectively. At the time I was shooting IDPA. I was shooting a fair amount and was rated Expert with a Glock 17.
I ended up getting the 26 after shooting them both because I could not double tap accurately with the .40. the recoil was enough more that I had to stop and reaim. To me, the ability to deliver two 9mm rounds within a couple of inches of each other a quarter of a second appart at defensive ranges was more important than the better balistics of the .40.
Don
p.s. I ended up selling the Glock and getting a Kahr P9. What a great carry gun if you are using a holster. If you're doing pocket carry check out the PM9. And they're made in MA.