I think you should buy a bike that reflects the majority of how you will use it.
If will be riding trails all day, then buy a mountain bike.
If you will mostly be on paved roads / sidewalks for short urban style riding, I would go with a "hybrid" or commuter style bicycle. This is basically a road bike with mountain bike handlebars and shifters. It allows for a more relaxed "heads up" style of riding. It is beneficial if you are navigating streets. However, many of these bikes are geared like road bikes and come with road bike tires so you can get going quickly on them.
If you won't be riding more than 10 miles at a time on flat surfaces I don't think it really matters what you get - I rode a mountain bike exclusively for several years while living in New Orleans. It was great to have a bike like that for the city because the streets were full of potholes.
Disc brakes are a nice feature to have, but unless you are commuting, riding in the rain regularly, or mountain biking, regular linear brakes are fine and likely are not worth the price premium.
I was recently looking at a nice hybrid bike with disc brakes. The one I wanted (Cannodale Synapse 2) was $1,100 plus tax at the bike store. Not very Mustachian, but a sweet bike.
I ended up finding a similar bike that was a few years old on Craigslist for $300. I ended up not getting a bike with disc brakes, but saved $800. Was worth it to me. Bikes like this are fun for tooling around the neighborhood, going to the grocery store, etc., but I also have been taking longer rides on the weekend (25 miles) with a small group and am able to keep up.