Penta-1,3-diene is more stable than penta-1,4-diene mainly because of

In penta-1,3-diene the p orbitals of the four sp2 carbons overlap across the central single bond, delocalising pi electrons and lowering the energy (the heat of hydrogenation is lower). Penta-1,4-diene has an $\mathrm{sp}{}_3$ $\mathrm{CH}{}_2$ between the double bonds, so there is no conjugation. Neither compound has cumulated double bonds.