I've never really noticed a difference in hardness so I am not usre what you mean; although , intuitively, I suppose, the coiled brake housing would feel harder. But differentiating between brake a derailleur housing is not a matter of opinion, and there is no need to make determination by feel. Look at the the cut end of the housing, if you see what looks like bunched strands all running in straight line, thats shifter housing; do not use it for brakes. If you see a single flat metal coil spiraliing away fromm you, thats brake housing. Also, standard brake housing in 5mm, shifter housing comes either 4mm or 5mm. So if you have 4mm shifter housing, it will be visually thinner than the brake housing. This method of differentiation won't work, of course, if you have 5mm shifter housing.

edited to add: dsbrantjr beat me to response, making same point with fewer words.