Easier and more cost-efficient than sanding walls?

I have 6 rooms in my house that have had some sort of work on the walls. In each room, the problem is the same - some walls are now textured, while others are smooth. Would it be easier to hang 5/8 in drywall over the whole house (I know for a fact that I can do that job) or would it be easier to sand the walls down (which I have never done before)? Also, which would be less time consuming, and am I looking at a huge price difference to do this?