You'd spent more time on Demon's Souls definitely, but that's not the only way you should measure value for a game. Heavy Rain will be much more memorable in the long run. It's genre-defining and much more engaging on a narrative level. Demon's Souls has very little story. And the whole "old-school hardcore style" gaming gets old quick.
In the end it just depends on your taste. Of course getting both games will be the best solution. I've got the platinum trophy for both games and have no regrets playing either of them really. But I just feel that Demon's Souls is a bit overrated, because while it's challenging and provides a solid satisfaction, it's significantly flawed in the story sector, which to me is too important to ignore.
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