It's the other way around. Imagine Lilith with another major demon's power on top of hers. Lilith being the driver is better than Mephisto IMHO. Mephisto has no qualms saying what he plans for humanity. I think Lilith, having had a major hand in creating Sanctuary and humanity in the first place, would have had at least some overlapping interests that would have worked in our favor.

It's like picking between one person who says they're definitely going to kill you and another person who says they'll maybe kill you if the need arises.