I honestly don't know much about relative salinities of terrestrial versus Martian prospective oceans. I do know however that everywhere that's been closely sampled so far by rovers and landers has had lots of perchlorate (Cl O4) salts in the soil, sometimes up to 0.5% of the mass. This can form when chloride salts react with surrounding minerals under the influence of ultraviolet light... nobody is terribly confident yet about what actually happened there to make them given that these results are new since the Phoenix lander and Spirit and Opportunity, but it's certainly interesting and suggestive.

I also think I should add that there is some evidence that a good chunk of Mars's water went underground - the topography of just about everything within ~30 or 40 degrees of the poles is indicative of crater walls slumping from shifting permafrost and there seems to be plenty of solid water in or under the soil there. The oceans may not have only dried up so long ago, they may have sunk downwards simultaneously.