See, if there are already many differences between DS1 and 2, I really don't see why a DeS2 would be so different from dark souls. I still want a true DeS2 sequel though, because I really liked the atmosphere and hub world of the first one.
Some prefer the base mechanics from Demons, I am one of them. I found the bonfire system tedious and, in principle, a little broken
There is also a huge difference in traversal of the world, given that Demons is a hub and Dark is one interconnected world.
They make for a different way of playing, certainly.
I played Dark before Demons, but enjoyed Demons considerably more. There are enough differences to make the experience not the same, and so people have a preference of which sequel they want to see.
I guess it would be natural to assume that a Demons 2 would use the demons mechanics, rather than the Dark ones, which some would prefer.