Also, explain to the gunshop what it's for. Most decent ones will cut you a break if it's for a police officer (or soon to be police officer).
As for my recommendation, check out Springfield XDs, a bit cheaper then glock and just as realible. I've carried mine for awhile now and shot it a lot. 0 problems.
If you really want a Glock, I like the Glock 19. Fairly compact, but still enough gun to be a good duty weapon. The 1/2 inch barrel difference isn't going to make a difference when it's time to qualify, the farthest we shot pistols in the academy was 25 yards, and thats no problem for a Glock 19. It's 9mm...but with decent ammo, it's just as good as any .40.
But it's really up to you lots of good options out there. Myself, I'd stick to the major manufactors, Sig, glock, HK, S&W, Springfield because they will most likely have the holsters and police accessories most available. But the bottom line is, just find something to fits your hand and has a trigger you like and you should be fine.