Thought into thirst

A lot of depths decks have been running this over thirst for knowledge.

Plunge into Darkness
1B
instant
Choose one - Sacrifice any number of creatures, then you gain 3 life for each sacrificed creature; or pay X life, then look at the top X cards of your library, put one of those cards into your hand, and exile the rest.

Entwine {B} (Choose both if you pay the entwine cost.)

Let me just say I side out at least one thirst every game, normally due to the fact I run into drawing 2 a lot. And 2 generally means Im holding one blank, by the time I would get the opportunity to play the second Im already dead. But does that mean I should play plunge? It costing 1 less allows you to do things like keep mana open for meddle much easier. Of course there is more to it than its mana, as you would need to pay at least 4 life at any given time to consider it taking up thirsts slot. Extended is a much faster format, and even fast combo decks like depths needs all of its resources to ensure it gets there. If playing plunge means setting yourself up for next turn bolt, tribal flames, 3-5 creature damage your more than likely dead vs zoo.