Ok, I'll answer the question!
The marketing hype would have you believe that the Venge and the Tarnic are two completely different bikes built to perform better in certain situations.
Tarmac better climber, less flex under acceleration out of corners, slightly lighter. So, if I was planning a summer trip to the Pyrenees, then the Tarmac would be the one to choose!
Venge more aero, faster in a straight line on flattish terrain etc etc... So, if I wanted to ride solo as fast as I could for as long as I could, then I'd opt for the Venge.
The caveat to the above is that we're really talking about very marginal gains, so, in reality, you'd struggle to notice much of a difference between the two; if you're looking for performance gains, switching from one to the other really would be a waste of money.
But, if you're happy to buy a mean, hard as nails looking Venge, and can afford it, then do so and enjoy it for what it is. Unless of course it's that orange thing, in which case the answer would be no!