Find out what your Department's approved carry list is and pick from there. Also, some departments have issue weapons that they issue to you to carry and then have approved lists so that you can buy and personally own your service weapon. I would go with whatever they issue or whatever is on their approved list.
It's a lot easier when you own or have a weapon that matches all the gear they issue to you i.e. holsters, etc... Years ago my agency issued Smith and Wesson 686 but you could carry Glock 22, Sig 226, and Beretta 96D's if you bought them out of pocket. Trouble was, they only issued holsters, etc... for the 686 so anything else you were out of pocket for. I would also see what caliber service weapons they issue before I chose any specific caliber based on its merits. I bet .40SW is a safe bet 95% of the time though. Every where's different but I hope this helps. It's been a while since any academies for me!
Also, as a current USBP firearms instructor, KX592's advise to "not to practice excessively before going" is very wise. You will have to unlearn bad habits. I was a self-taught pistol shooter and had a heck of a time becoming more than a mediocre shot because of all the recreational practice shooting I had done and the subsequent bad habits I had formed. It really took a lot of undoing and repetitions with good habits to finally shoot distinguished expert. Not so with the rifle as I had been formally trained.
It's always easier as an FI to teach good marksmanship to a person who has no shooting experience than a person who has done a lot of informal shooting or who has had inadequate training.