The reason to go diesel is you are pulling heavy and pulling a lot. If you are in the mountains, its also a plus because the gas engine without a turbo will lose a lot of power at 6-10,000ft and with newer trucks you get an exhaust brake with the diesel for going down those hills.
You hit all these points in favor of diesel. 15k trailer is big and you sound like you are going to pull a lot and in the west implies a fair bit of mountain towing.
PS: I would want a 1 ton dually if going that big but check the actual weight ratings. I doubt a 3/4 ton will have enough once fully loaded.