Let S k (N , χ) be the space of cusp forms of weight k, level N and character χ. For f ∈ S k (N , χ) let L(s, sym 2 f ) be the symmetric square L-function and L(s, f ⊗ f ) be the Rankin-Selberg square attached to f . For fixed k ≥ 2, N prime, and real primitive χ, asymptotic formulas for the first and second moment of the central value of L(s, sym 2 f ) and L(s, f ⊗ f ) over a basis of S k (N, χ) are given as N → ∞. As an application it is shown that a positive proportion of the central values L(1/2, sym 2 f ) does not vanish.