although glocks have more in common with an SA gun than a DA gun many people consider them DAO and that does have the letters DA in it so maybe it would be ok.
i bet many departments that allow DA would also allow a glock.
with that said i would choose a Glock 19. Normally it would be a toss up between a glock and beretta but it has to be for off duty carry also.
I like the .357sig round also but i don't think its much of an improvment over the 9mm and i like the 9 for many reasons.
most reasons why i would choose the G19 have been stated already but i'll list some more:
1. firepower. 15+1 rounds of 9mm 124gr+P Speer Gold-Dots.
2. concealability. the G19 is big enough for duty carry (many people do it including NYPD) but can be easily concealed. it is much lighter than other compact guns like the sig 228 and also is slimmer and easier to hide. also it just barely reaches the barrel limit but is over 4".
3. reliability. glocks IMNSHO are the most reliable pistols made (maybe berettas tie with them for first). my G19 has proven 100% reliable since i bought it. there is no pistol i trust more than my G19.
4. Accuracy. many people say glocks are inaccurate but that is from a benchrest. sure sigs and other guns will outshoot glocks from a benchrest but LEOs don't shoot from that position. Glocks are more than enough combat accurate and i can shoot mine more accurately than i can shoot a sig or beretta.
5. Feel. part of why they are so accurate for me is their feel. the G19 fits my hand very well and points naturally for me. i can get it on target much faster than other guns.
6. trigger. i love glock triggers. they are so much easier to shoot than DA or DAO guns. the first day i bought my G19 i outshot by berettas with it. that was because of the trigger. the first DA shot on my beretta 92 always was a problem for me and the first shot is the most important. i really need every shot to count and i can do that with a glock.
7. easy to use design. i don't want to have to fumble around with safeties when i need my gun for defense. i want to be able to draw and fire it as fast as possible and with as few steps as possible. i can do this fastest with a glock than anything else.
8. durability. not that big a concern but most LEOs have their guns for quite some time and i want it to last a while. One of my friends bought a sig 226 from an LEO who had it a while. the gun does not function properly now and jams all the time since its too old and worn out.
9. I really like my G19!