Let X be a symmetric quasi-Banach function space with Fatou property and let E be an arbitrary symmetric quasi-Banach sequence space. Suppose that (f k ) k≥0 ⊂ X is a sequence of independent random variables. We present a necessary and sufficient condition on X such that the quantityadmits an equivalent characterization in terms of disjoint copies of (f k ) n k=0 for every n ≥ 0; in particular, we obtain the deterministic description of n k=0 f k e k ℓq Lp for all 0 < p, q < ∞, which is the ultimate form of Rosenthal's inequality. We also consider the case of a ∆-normed symmetric function space X, defined via an Orlicz function Φ satisfying the ∆ 2 -condition. That is, we provide a formula for "E-valued Φ-moments", namely the quantity E Φ (f k ) k≥0 E , in terms of the sum of disjoint copies of f k , k ≥ 0.