bike upgrade question

The P2C is a more aerodynamic frame, so there may be some efficiencies to be gained there. But the primary determinant would be your ability to adopt and maintain an aerodynamic position that allows you to maximize your power. Can you do that better on the Motobecane or the Cervelo? Depends.

Another factor to consider: would you enjoy training on one more than the other, and thus train more?

Which would you enjoy riding more?

Which will make feel faster on?

Which fits you best?

I think for sure you should get a new bike to gain a few seconds...new bikes are fun.

Dura Ace will have no effect on aerodynamics or speed, so take this out of the equation (and save some cash and lower your sights on the Ultegra model for starters). 

An argument can be made that the Motobecane properly fitted with the rider in an optimized position, a set of race wheels with a disc rear and aero helmet will have more benefit than just a P2C frame and save you some money. 

But I'll agree that buying a new bike is a definite boost to the psyche and will make you feel faster!