Neither! Sulfate does not absorb or emit radiation so it cannot be resolved, thus a saturated Barium Chloride solution is usually used. However, Sulfur can emit in the far UV so vacuum ICP can be used.
The method depends on the concentration of sulfate ion in your water sample. If you think that the concentration is in sub ppm range then go for Ion Chromatography.
If you think the concentration is in percentage level then use the Barium Chloride method.