Adding the basement to the living space adds more value because you can include that basement in the square footage of the home. You can even rent the basement to a medical student (they are usually quiet renters and are mostly found studying at the library), for a semester or more.
I agree, both basements and attics are great assets if they are finished and ready to use, they add a lot of value.
It's always worth it to finish any construction or repairs you're able to in your house, especially when you're looking to sell. Sometimes something as small as a $1000 repair can add $10k value to the final price of the house, at least that's my experience from when I was working in real estate.
