We compare the cost complexities of two approximation schemes for functions f ∈ H p (Ω 1 × Ω 2 ) which live on the product domain Ω 1 × Ω 2 of sufficiently smooth domains Ω 1 ⊂ R n 1 and Ω 2 ⊂ R n 2 , namely the singular value/Karhunen-Lòeve decomposition and the sparse grid representation. Here, we assume that suitable finite element methods with associated fixed order r of accuracy are given on the domains Ω 1 and Ω 2 . Then, the sparse grid approximation essentially needs only O(ε −q ), with q = max{n 1 , n 2 }/r, unknowns to reach a prescribed accuracy ε, provided that the smoothness of f satisfies p r((n 1 + n 2 )/max{n 1 , n 2 }), which is an almost optimal rate. The singular value decomposition produces this rate only if f is analytical, since otherwise the decay of the singular values is not fast enough. If p < r((n 1 + n 2 )/max{n 1 , n 2 }), then the sparse grid approach gives essentially the rate O(ε −q ) with q = (n 1 + n 2 )/p, while, for the singular value decomposition, we can only prove the rate O(ε −q ) with q = (2 min{r, p} min{n 1 , n 2 } + 2p max{n 1 , n 2 })/(2p − min{n 1 , n 2 }) min{r, p}. We derive the resulting complexities, compare the two approaches and present numerical results which demonstrate that these rates are also achieved in numerical practice.