In my opinion, Demon's Souls is much better than Dark Souls. It provides much more clear cut direction than Dark Souls and each section of the game seems to have much more character than the areas in Dark Souls.
And if for no other reason, Demon's Souls will cost less, have more direction and by the time you finish with it Dark Souls should have a price drop. Its just makes more sense fiscally.
If you do buy Demon's Souls, consider buying the EU version, from what I've heard the EU servers are more populated than the US ones, but in the end it shouldn't make much of a difference.
And if for no other reason, Demon's Souls will cost less, have more direction and by the time you finish with it Dark Souls should have a price drop. Its just makes more sense fiscally.
If you do buy Demon's Souls, consider buying the EU version, from what I've heard the EU servers are more populated than the US ones, but in the end it shouldn't make much of a difference.