Who would the girls be dead without? Spencer v. Mona

There are two very smart girls on the show that truly do save the day: Spencer and Mona. Spencer Hasting is such an strong individual that uses her wisdom and erudite ways to constantly save the girls. But, right on Spencer's tail with the gift of knowledge is the infamous Mona. Mona is quite the character for she has the background story of just some loser trying to find her place in the world while having horrible encounters with malicious people. However, her come around is quite extraordinary in the sense she messes with the girls lives now but playing mind games. Is she really there to help Aria, Hanna, Ali and Spencer? Mona deems her allegiance by helping the girls find out who A.D. really is. But the question is, would the girls really be more dead without Spencer or without Mona?