Tasha Lem is the cause and River Song is the effect.
The Mother Superious of the Papal Mainframe called out the Silence, unwittingly setting Kovarian on her jounrney into the “long and bitter war” against the Doctor, which ended up causing the cracks in time in the first place. River is a product of this very conflict. She is a chess piece in this war, it runs in her blood in the most literal way – genetically manipulated, conditioned from birth on to fulfil her purpose.
It is only fitting that they would share similarities, but they are also profoundly different people.
What River and Tasha have in common is a considerable potential for destruction
Tasha, the religious leader. She deals in power, both in ideational and physical. There’s a visible thrill as she assesses the situation at Trenzalore and cautiously wages her influence to tip the scales in favour of her purpose. We see just how perilous this can be as her decisions become a catalyst to the dangerous situation she is attempting to salvage.
River, the former assassin. Strong, cunning, rebellious, raised to be a killer. But even after she decides to make her own way, to go against her conditioning, she still made time stand still when she refused to be used again. She’s the woman who believed her suffering to be greater than that of a dying universe, who led the world to a brink for love.
The difference lies in the path they take, how they react to an awareness of just how much harm they could potentially cause: While Tasha turns to peace and doctrine, River finds herself in knowledge and tempered chaos.