While I don’t recommend Glocks since the safety is on the trigger, meaning the trigger is the only safety, they are reliable and I have owned two with no problems. I would recommend going to a shooting range that rents them and try a few. I would consider the 9mm as the least powerful round to consider (Glock 17 is full sized 9mm and Glock 19 is smaller). Going up in power are the .40 cal S&W, 45ACP and 10mm. The 10mm is most powerful (Glock 20 is full sized and 29 is smaller) but ammo is more expensive and recoil makes them more difficult to shoot. I am not familiar with the 40 cal Glocks, but this round is almost up to 45ACP power levels. In 45ACP there is the full sized Glock 21, the smaller Glock 30 and the compact Glock 36.
I have owned the Glock 19 9mm and the Glock 30 45ACP. If you can handle the recoil I would go with the Glock 21 or 30 in 45ACP. If not, I would go with the Glock 17 or 19. I mention these two calibers because the ammo is more common and is cheaper, giving you more opportunity to practice. For self-defense loads use a premium hollow-point if you don’t live in NJ. Good brands are Speer Gold Dot, Federal HST, Winchester SXT, and there are others. For practice use less expensive full metal jacket (FMJ), but make sure you shoot a few boxes of the home defense choice to make sure it will feed reliably.
You may want to try other guns you can rent too. A 357 magnum revolver can shoot 38 special ammo as well and reliability is usually better than with a semi-auto pistol. I personally like a thumb safety on a semi-auto. Many quality manufacturers offer guns that are similar to Glocks, but with a real safety feature.
I'll echo 'Climbers comment here. When my wife decided she needed a CCW we went with a J-frame .38 due to her experience level and weak grip. I retired my autoloader for a J-frame 357 and carry with magnum Hydroshocks. When traveling together I retain the magnums but we stock our speedloaders with 38+P Hydroshocks since both revolvers can use the common rounds.