Any suspension mtb with knobbies will be slower feeling and sluggish on road commutes. Locking out the fork doesnt make soft knobby tires roll fast and smooth.
An adventure bike, either flat bar or drop bar, could work well and play double duty. I can ride my monstercross on our local flowing singletrack and have fun, though I couldnt do other MTB activities like downhill or jumping. I can also ride my monstercross on the road and it is basically as fast as I ride my road bikes. Some quality mixed tread 40mm tires can roll smooth on pavement but be large enough to absorb bumps on light rough singletrack and still have fun.
Most every large company has an adventure bike or three in their lineup at this point. Flatbars and drop bars exist.
With that said, if you are set on an MTB only, I would go rigid every day of the week. According to some here you will come close to death by riding a rigid bike on trails, but it isnt close to as bad as they make it sound.