Does your dept have any stipulations on make/model/features? if you don't know, ask before you buy.
I'd opt for a full size Sig 226 DAK in .40 before the 250. The 250 has been dropped by a few agencies. Yeah there are going to be people that like them and swear by them but when multiple agencies with a broader range of samples drop it...none for me thanks.
Make sure to ask about agency discounts, such as the Glock blue label program etc.
The S&W M&P is a good choice. There are not many agencies that issue XDs as duty pistols. Sigs, M&Ps, and Glocks are your big three for those, add HK in as well, you should be able to find some good used HK USP .40s out there for a pretty good price if you look hard.