First of all, welcome to the forum and congrats on going to LE academy.
One of my roomates just became an Orange County Deputy but he was living with me while he was in police academy (training). He told me their department gave each officer a choice of either Glock21 or Glock22. He picked the G21, but he is an excellent shot anyway being a veteran Marine.
Given the choice between 40 and 45, you really can't go wrong with either caliber. I'm sure people have their preferences and biases, but go with whichever you shoot better. 40 will give you a couple extra rounds, but 45 puts "bigger holes" if that matters.
In the end it will just boil down to your preference if your department gave you a choice between different calibers. Look into what's issued first, as they may just issue one caliber instead. If you also choose to buy your own duty weapon, you must qualify with that weapon before you can carry it on duty.
Best of luck!