^exactly what he said, since u have a 9mm just get a .45, as for the .40 n .45, they r both great rounds, the .40 is great for self defensive, the 45 jst packs a lil more punch in my opinion.
Heres an exmaple, during the summer season people wear less clothing so a 40 rnd will do the job as in winter people were more clothing so a 45 will penetrate deeper through the clothing and into the person.
It really depends on you.. I have a Glock 19 GEN4 its good, I also have the 26 GEN 4 and the 17 GEN4, there all good for 9mm..
It all depends on what round your comfortable with .40 and .45 are more stoping power but rounds are kinda expensive.. Plus if you use them to go to the range there not that accurate at more then 100 to 150 yards look in to it, check with your local range and buy some time at the range see if they have some that you can shoot there, that might be an option it will also help you decide which one you really want...