In summary, we obtained an unprecedented high resolution mapping of the hydration dynamics around analogous ds-RNA and ds-DNA 18-mers, which allowed us to highlight, beyond apparently similar features with an average moderate slowdown, key differences. We rationalize these differences and obtain a physical understanding of their molecular determinants thanks to an analytic jump model. In RNA, the slowest water molecules belong to the phosphate hydration, while they are located close to the bases in the minor groove in DNA.