Put me in the 'either/or? why not both?' category. My current commuter/distance bike has an Ultegra front crank and derailleur and Deore XT rear. So, get the light (but still durable) road crankset with the integrated bottom bracket, but still have the option to rock a wide range of mountain bike gears with the rear derailleur. Best of both worlds!

Oh, and for what it's worth, the 105 group is plenty durable. Prior to getting the Ultegra/DeoreXT hybrid, I owned a Trek 520 touring bike with a mixed 105/Deore drivetrain, and I've used it for year-round commuting in the Northeast, light touring, randonneuring as well as off-road centuries and fire road riding in Maine. The components took it all like a champ.