Attic VS Basement- better value add?

[Jack Robinson]

Would you rather finish out a basement or an attic? 

I work as an investment agent finding distressed properties investors can add value to. One of the most common ways I see pros increase equity is by finishing either the top or bottom of a house, yet there does not seem to be a consensus of which produces better returns. Are there specific systems designed for the upper part of a house which aren't applicable to the lower half? Curious to hear if people find more success with finishing out attics or basements.