The duty gun should be your last worry right now. As has been stated, there is a TON to worry about before the specific firearm comes into play.
My department allows me the freedom to carry pretty much what I want...we have a large approved firearms list.
I carried a Glock 17 for a long time, but now it is mainly my M&P .45 that rides with me. Thinking about picking up a full size M&P .40 since I already have the compact...they will make a good primary/backup pair. I have an LCR in .357 that I carry as backup right now.
"They're on our right, they're on our left, they're in front of us, they're behind us; they can't get away from us this time."
Chesty Puller, USMC, Chosin Reservoir, Korean War
"You cannot exaggerate about the Marines. They are convinced to the point of arrogance, that they are the most ferocious fighters on earth - and the amusing thing about it is that they are."
- Father Kevin Keaney, 1st MarDiv Chaplain, Korean War