We report crystal magnetic susceptibility results of two $S=1/2$ one-dimensional Heisenberg antiferromagnets, ${\mathrm{KFeS}}_{2}$ and ${\mathrm{CsFeS}}_{2}.$ Both compounds consist of $({\mathrm{FeS}}_{4}{)}_{n}$ chains with an average Fe-Fe distance of 2.7 \AA{}. In ${\mathrm{KFeS}}_{2},$ all intrachain Fe-Fe distances are identical. Its magnetic susceptibility is typical of a regular antiferromagnetic chain with spin-spin exchange parameter $J=\ensuremath{-}440.7\mathrm{K}.$ In ${\mathrm{CsFeS}}_{2},$ however, the Fe-Fe distances alternate between 2.61 and 2.82 \AA{}. This is reflected in its magnetic susceptibility, which could be fitted with $J=\ensuremath{-}640\mathrm{K},$ and the degree of alternation, $\ensuremath{\alpha}=0.3.$ These compounds form a unique pair, and allow for a convenient experimental comparison of the magnetic properties of regular versus alternating Heisenberg chains.