When you say you're in training to become a cop, do you mean:
A. You have been hired by an agency, and are about to start their academy?
-or-
B. You are about to put yourself through an academy and start applying to agencies?
If the answer is A, do the following:
1. Find out what guns the PD will allow you to carry on and off duty, and whether or not they provide the weapon for you. Most agencies will stipulate a duty gun (or allow you to choose from several) and provide it. Back-up and off duty guns are often allowed from an approved list.
Each agency is different, however. Find out.
2. Make a purchase decision based upon the info gathered in #1. If you're stuck buying your own weapon, might as well buy what you're going to be carrying. If they buy the duty gun for you, buy one you can carry OD or as a BUG, so as to get trigger time with something you'll be using.
3. Spend some time and money on professional training. Most ranges offer beginner courses. Take advantage of this.
If the answer is B, do the following:
1. Find out what guns the academy(ies) you're considering will allow you to bring to school. Make sure whatever you choose meets their criteria.
2. Go to a range, rent a lot of guns. Pick the one that meets the criteria for LE carry/training, and that you shoot best or feels best to you, and buy it. "LE training/carry requirements" are usually something like "must be in a standard caliber" (think 9mm, .40S&W, .45 ACP for autos and .38spl or .357 magnum for revolvers) and of common/standard make (think Glock, XD, S&W, Sig, H&K, etc). Now is not the time to get a less common make/model/style of gun or a less common caliber- you're trying to get practice with a weapon that is generalizable to many other types. A CZ-52 in 7.62 Tokarev, while a great bargain, does not really help you.
If you're unsure about specific makes/models/prices after shopping and trying, post back here and someone will be able to fill you in.
3. Spend some time and money on professional training. Most ranges offer beginner courses. Take advantage of this.
Don't skimp on the training. Every academy class my agency puts through loses at least one recruit in the Firearms stage. While the vast majority of people can step in as a novice and do okay, some people need a little extra help. You won't know if you're one of those people until you try, and failing out is a hard way to discover it.
Two pieces of advice on surviving academy: Never ever lie, never ever give up. They push you to see what you'll do under stress. If your reaction is to lie/cover-up, or fold like a piece of paper, they'll wash you out in an instant. Train hard, be honest, and never ever quit. Lie in the academy, under contrived pressure, you'll be a liar in service under real pressure. We're better off without you. If you quit in the academy, you'll quit on the street. Quit on the street and someone can die; either you, a fellow officer, or someone you've sworn to protect. Stay in the fight!
Mike