My advice - give your circumstances and your husbands trade - is for an attic conversion. He will likely be able to do most of the work himself regarding reinforcements, studs, boards, flooring etc. So will work out cheaper for you (but might stress the marriage!)
As before however, if you do not have the required clearance, you will not be able to classify it as an extra habitable room - so it will not greatly enhance any resale. Ecape routes and access are key to ensure safety for any attic conversions.
All things being equal, the attic conversion will give you more bang for your buck.