Both.
But Sanji rejected science when he destroyed the RS, so everyone saying he's going to be power of science with next upgrades is an idiot. He never accepted science, what he accepted is that he couldn't change what already had been done to him so he may as well use it in addition to Haki. But any further science was rejected as he flat out stated he'd win his own way.
So no,
Science isn't replacing Haki for Sanji.