I was thinking of something bigger in a semi-auto handgun than my 9mm. I was looking at .45 acp and found with a kit upgrade, can shoot .45 super. On the other hand the 10mm can shoot .40 S&W to save money when just target shooting. Any preferences?
If you are considering the .45 Super, I would suggest looking at the .450 SMC; it has a stronger case and uses small rifle magnum primers. Only use in guns that support +P, and fully support the case (important for the Super); the chamber pressures are pretty significant. Shooting it the .45 Super or .450 continually can take a toll on the frame in some guns. With some guns, switching between .45 ACP and the .450 SMC it Super may require a change in recoil spring to run reliably. When in Brown Bear country fly fishing, I carry my HK45 loaded with .450 SMC with hard cast solids from Double Tap. Fortunately, I have never had to engage a bear... I like them and prefer mutual avoidance (I do carry bear spray as I do when in AK). I would be comfortable carrying a G20 as well; some of my AK friends have taken to doing that.
I hear tell that that 10mm has more penetration and et2t. But i dont really know much about either. I'd prolly go with .45 purely on ammo availability.
My suggestion would be a good high quality 45. You can practice with standard velocity ammo, but you can load it with 45+P ammo for carry. I suggest Federal or Remington 185 grain which has a muzzle velocity of 1140 fps, which is comparable to the 9mm. If you've never shot 45+P ammo out of a 45, let me say that it is pretty "dynamic", and you ought to try it. If you're looking to be able to launch bigger rounds than a 9mm at about the same speed, this is a good way to do it.
If the only choice were between the two I'd go with .45 simply because of ammo availability. 10mm is the first cousin of .40, and I'd choose .40, again, simply for ammo availability.
You don't want to shoot 40 from a 10mm auto as you are relying on the extractor to hold the case in place. 10mm is, in every way, 40 magnum, with considerably more energy than 9mm 40 and 45ACP - it's on the high end of .357 magnum. If you can handle the recoil and want to carry 10-12-16 rounds of hot .357, get a 10mm. I have 2.