Most people don't want (or have enough ammo) to practice enough to get proficient with a 45. They are also larger and harder to conceal. Regarding anything less powerful than a 9x19 I'll quote Blazing Saddles "Don't shoot him--you'll just make him mad". Read up on the reliability of the various 9mms and shoot some if there is a shop where you are able to do that. It should point naturally in your hand, have a good reputation for reliability, and be within what you consider affordable. For reference, if you can test a Glock 19 you will know a little about what direction you want to take.
How would being proficient with a .45 ACP be any different than other calibers? It has a lower recoil impulse than 9mm so for a lot of people with the increased mass and weight of the gun it is actually easier to shoot.
I find this to be the case as well. At 15 feet I stack rounds to the point I can target repair stickers from greater ranges.
A lot depends on the load of course. The people I have trained are generally more comfortable with a 9, even a lighter one, than a 45. It's all very subjective. I think they are more comfortable with the 9 and thus shoot better earlier. Just one guy's opinion and what I suggest to students initially.
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