Happily, your department will put a lot of time and money into training you in all sorts of things,including the use of your handgun. And yours is the pragmatic approach of a master craftsman regarding his tools, or an Olympic athlete regarding his performance: get the very best that works the very best for you.
Best approach for right now: dab around a bit and try out some of the suggestions that have appeared in response to your post here,but don't go overboard- it's just familiarization you're looking for right now. Then let your department train you in whatever standard or approved weapon that agency uses, which is hopefully based on what has worked for other officers in the past more than on budgetary considerions or outright kickbacks- which has been the case too often in my area.
Then, after you find yourself as a rookie/probationary spending lots of time with your field trainer, find out what's worked for him/her and others using the same equipment you use in the conditions of your local area.
Most likely you'll fimd that it's more common for agency-issued holsters, belt equipment and ammo to be crap rather than the weapons themselves- and you may find it smart to practice using your own magazines that ALWAYS work [or else get rid of them] rather than the pass-around leftovers that others have had problems with while they're on the range. And, like your cuffs, your personal mags should have your badge number engraved on them after you've got one.
I've ownned most of everything in the Glock line, most of which just doesn't fit my hand well, and I don't care much for their .40 caliber offerings. If the city you work for gives you a free one, swell. Likewise I find the issue M9/M96FS Beretta that I was stuck with for deserter apprehensions and transports to be more than a little clunky in my habnd, though the California Highway Patrol was still using the.40 version of them, last I heard. I've owned and carried 9mm Browning GP/ Hi-Powers since 1967, and have one I've owned since 1968; it's had just over a hundred thousand rounds through it and has needed rebluing three times. I had it during one critical shooting incident and it got the job done though I've changed my thoughts about the ammo for that use since then...but I very much know what that pistol will and won't do, though I've more-or-less retired it from serious use.
And most recently I fall back on my own beginnings and use a .45 ACP Government Model, most recently an Argentine Modelo 1927, sometimes a .45 Combat Commander. The M1911 design has served my family well for three generations and is about to enter its fourth, so in part I'm setting an example that may be followed in part or in whole, or may become the basis for an informed choice to go with something else.
But in general, I'd advise you to first pick the caliber you'd most prefer to have your life depend upon. You can then choose which mechanism in which to use it.
