Poincare was confused on several points (see the discussion on Wikipedia regarding "mass energy equivalence"). He could never get the mechanical relations straight, since he could not figure out that $E=mc^2$. Einstein followed Poincare closely in 1905, he was aware of Poincare's work, but he derived the theory simply as a geometric symmetry, and made a complete system.
Einstein did share the credit with Lorentz and Poincare for special relativity for a while, probably one reason his Nobel prize did not mention relativity. Pauli in the Encyclopedia Britanica article famously credits Einstein alone for formulating the relativity principle, as did Lorentz. Poincare was less accomodating. He would say "Einstein just assumed that which we were all trying to prove" (namely the principle of relativity) (I could not find a reference for this, and I might be misquoting. It is important, because it shows whether Poincare was still trying to get relativity from Maxwell's equations, rather than making a new postulate--- I don't know).
Special relativity was ripe for discovery in 1905, and Einstein wasn't the only one who could have done it, although he did do it best, and only he got the E=mc^2 without which nothing makes sense. Poincare and Lorentz deserve at least 50% of the credit (as Einstein himself accepted), and Poincare has most of the modern theory, so Einstein's sole completely original contribution is E=mc^2.