I am trying to improve my Legacy control black deck. it works really well but there is always room for improvement.

The Deck: https://deckbox.org/sets/952940

My question is which would be a better fit in the deck?

Phyrexian Negator vs. Phyrexian Obliterator

Pros for Phyrexian Negator
First turn drop with Dark Ritual
Can be brought back with Unearth, best side board against heavy creature destruction

Pros for Phyrexian Obliterator
Opponent has to sack instead of me
Opponent wont usually block
Can be used as a blocker to slow down opponent

I would take obliterator over negator almost anyday. While negator was good back when i first started playing, it really needs to have a deck build around that concept with it ( mindslicer, abyssal gatekeeper, and so on...) where obliterator just rolls with the punches.

As a person who played mono-black in Urzas I can tell you that Turn one swamp, dark rit, Negator go was the best feeling in the world. Then when your opponent dropped a mountain and bolted it and you had to sac your land you were time walked for one and now on a mull to at most 4.
I'd go the safer route with Obliterator.