I'd say it was almost the reverse of what was originally said... Pothead is someone who uses pot regularly, and is a part of their life, whether good or bad (the implication is definitely negative though). A stoner is just someone who regularly gets stoned... on anything. Someone just doing things to get stoned would care less about outside responsibilities and be less productive overall.

I'd rather be a pothead than a stoner... but just "smoker" would be a much more appropriate label for many people!