Go through Demon's Souls first and then dive into Dark Souls. You will appreciated them both for what they bring to the table during their release period. --- Effort gives confident in oneself as Pride does not equal ability. ~DemonSoulLoki
Demons is a tighter, more controlled game. There are fewer opportunities to stockpile good gear earlier on and fewer opportunities to seek the most "do-able" area. As a result, it ends up feeling much, much more difficult. Even as a Dark Souls veteran, I'm afraid to revisit Demons' Souls. As such, for the horror of the experience, I would suggest playing Demons first.