Dirt drops are great for singletrack riding. They're not so great for medium and long-distance rides, because they really only offer 2 useable hand positions (tops and drops). I only have experience with Salsa Woodchipper bars, but the hoods were completely unrideable. Also, with dirt drops, the top of the bar needs to be higher than the saddle in order to provide a comfortable ride. If the bike's not setup properly, the ride can be quite cramped.

I have no experience with moustache bars, but my gut tells me that it's the better setup for mountain bike conversions because of the way they're sized. People tend to buy their mountain bikes a little smaller, so the extended reach of the forward position on the bars is probably going to make for the most comfortable position. The rear sweep also gets you upright for climbing, or riding that odd bit of singletrack.