Sorry for the ignorant question but what is so special about 10mm? Is it a lot more powerfull than 9mm? How would you rate them when compared to .45?
That is a very good question, actually.
The short an answer, as has already been described, is that the 10mm is special because it gives you the ballistic equivalent of a .357 Magnum in an auto pistol. My Glock 20 holds 15+1 rounds. Each one of them is could be a 135 gr JHP @ 1600 fps all the way up to a 200 gr JHP @ 1250 fps or a 230 gr WFNGC @ 1120 fps. As it just so happens, mine is loaded with Double Tap's excellent 180 gr Gold Dot load, with an advertised velocity of 1300 fps. This gives me a total capacity of 16 rounds, which happens to be about 2.5x the firepower of your typical .357 Magnum revolver. That is a lot of firepower for a pistol.
The 10mm Auto, like the .357 Magnum, also happens to be extremely versitile. You can get FBI-Lite level loads that match the ballistics of the .40 S&W, but usually with less recoil than a similar load in a smaller .40 S&W handgun. Or you can drive those bullets 150 to 200 fps or more faster than anything you can safely duplicate from a .40 S&W with similar barrel lengths. This is esp true when bullets of 200 gr or heavier. Most .40 loads stop at 180 gr cause it doesn't have the capacity to push anything too much heavier at a useful velocity. With the 10mm, you can go all the way up to 230 gr, which makes the 10mm Auto a useful short range hunting pistol for medium game up to the size of mule deer or black bear. If you do a lot of hiking, you can strap on a Glock 29 and have 10+1 capacity of 200 gr Hornady XTPs, which even out of the shorter barrel will usually manage 1200 fps or more, in a compact, lightweight package. This load would be an excellent choice for defense against people as well as cougars or a rogue black bear. When you get back into town, you can load a 165 gr Gold Dot out the barrel at about 1400 fps and have an excellent load for two-legged critters.
The 10mm Auto is capable of being loaded to meet or exceed everything the .45 ACP can do save for inital diameter and, in most defensive loadings, initial weight. It carries more energy, more momentum, and more sectional density, which means it is more likely to expand and holds more potential for penetration, if that is what you are after. I look at the 10mm Auto as the illegitimate love child of the .45 ACP and the .357 Magnum, because it combines .357 Magnum energies with bullets of larger diameter and heavier initial mass. It really is a great cartridge that deserves far more popularity than it has, and I find that most people who experience it first hand, really fall in love with it. The 10mm Auto has managed to form a pretty devote fan base. In fact, between you and me, sometimes it is borderlined cultish.