100% duty belt, external holster. At the moment I won't be carrying concealed. Personally I have used Glock before but have no experience at all with other brands and weapons, except a Springfield 1911 which I can't use.
I think the only reason to go with the Glock 19 over the 17 would be if you were going to carry concealed. It's common sense that up to a point, a similar but larger gun shoots better. I have shot the 19 a few times at the range, and it seemed like a good gun to me, but nobody at my IPSC club uses the 19 over the 17.
For what it's worth, at least one guy I know likes the M&P over the Glock, again for IPSC. It should be solid enough.
If none of the three guns has major reliability issues, I'd just choose based on which gun gives you the best grip. It's not good enough to try them in leisurely range shooting, you have to do fast draws, fast repeated shots, etc. and see if you still have a perfect grip afterwards for an accurate shot. For instance, my fancy CZ75 SP01 Shadow seems to fit my hands well, but with a symmetric grip that I'd like to use for speed & accuracy, it twists itself into a worse grip when shooting fast, and to prevent that I have to squeeze the gun really awkwardly and that costs performance. I should have gone with a Glock to be honest. It didn't feel as good at first but for my hand it does the important thing, locks into the grip. The Glock also points better for me (although I consider pointing completely secondary to grip stability - you can always tilt the wrists a little bit and drill that to muscle memory).