What are the differences between the two version of ARK?
Solution 1:
As you said ARK: Survival Evolved first cam out on June 2015 which was about building bases, exploring and surviving as long as possible.
ARK: Survival of the Fittest is a free to play Arena game, there can be up to 72 players in an arena. In the arena you have to kill everybody before someone kills you, until you are the last survivor (its kinda like Hunger Games). It was actually made for a Tournament where you could win up to 10000 $
The negative reviews mostly are because of the bad performance SOTF has.