Hey Todd,
First of all, sounds like a really cool job! I don't think you could go wrong with any of the suggestions made so far. When I read your comment about dirt and grime being a factor, I immediately thought Glock.
I have a G36 which I bought because I wanted something that wasn't going to weigh me down on the hiking trails etc...I was a bit skeptical at first, but for me it's been a real pleasure to carry. If capacity is a concern, you could certainly go with a different model, like the G30SF or even a 9mm. I carry mine in a Galco King Tuk, and it's very comfortable, of course YMMV. While I do not own one personally, I have heard mostly great things about the Crossbreed line of holsters as well. Comfort was a major factor for me.
Lastly, and someone may have mentioned this already, a good gun belt is a must. I use belts from Wilderness tactical and The Belt Man and both have lived up to their reputations.
Good Luck,
Bobby