Ian,
Will your department buy you full-cap G22 mags? If not, I just want to warn you that pre-ban Glock 22 mags are hard to find and expensive. Also, ammo is more expensive to buy for practice. That's not to say the Glock 22 is now a bad choice, it isn't. The Glock 22 is an excellent weapon. My point is that the more of this that you have to buy, the better off you'll be with a Glock 17. Pre-ban mags are more plentiful and less expensive, 9mm practice ammo is cheaper, and as far as the old "9mm vs. .40 S&W" argument, either is an excellent round. High performance LE rounds are equally effective in either caliber. I say this because I own and shoot Glocks in both calibers.