Low cost?
It's been commented before a dedicated NEO space telescope is even cheaper and that you don't need the moon for NEO detection and observation. Maybe the Far Side is useful for radio astronomy - and that's about it, as far as I can see.
As for a choice between Luna and Mars - obviously Luna, it's closer. Apparently, getting back from Deimos or Phobos (but not Mars obviously with its gravity well) is cheaper in energy terms than even from Luna. What that means - who knows?