If you are using salt in your softener the sodium ion replaces the hardness ion in the water. You get sodium in the softened water.
Reverse osmosis filters may have a limit on the TDS of the raw water that they can handle.
Exactly.. if you're on a low sodium diet your doctor isn't going to be happy with you.
Our TDS was 230 ppm, which is pretty low by most standards. The Canadian limit for sodium is 300 ppm in drinking water, and most doctors with an eye on your blood pressure like to see something in the order of 20-30 ppm sodium level.
The salt softeners do exchange hardness for sodium, as Egon said, and in most cases it isn't a problem. Ours would have been in time.
The no-salt softeners don't do well with water that has high manganese or TDS, according to our installer. If you have simple hardness problems (few do) they are fine. What they do is alter the molecular structure of the hardness (usually calcium) so that it doesn't attach itself to pipes, fixtures, etc.
The reverse osmosis looked after the sodium content in our softened water, and also produces the best tasting water I've ever drunk.
We also had concerns with dumping the salt brine and backwash into the septic system, it will form deposits over time that can clog the system. We dump ours outside via a gravity drain. Not a perfect solution, but not bad considering the tons of road salt the municipality dumps on our roads every year for snow and ice.
Sean