LOL Cost is a huge difference in and of itself. Efficiency is another... salt being more efficient. The amount of sodium added to the water depends on how much hardness it had to remove. Unless you are on sodium restricted diet, I would not worry about it. If you are worried, use an RO filter to remove the sodium.
Sodium. Potassium not only cost a lot more, it also should be set up differently than sodium. It is too technical to go through right now (I have to get to my daughters Hockey game), but... potassium is not a good idea in general.
RO water is really nice to drink. Once you try it it is hard to settle for less. I have taste tested my carbon-filtered, softened water against the RO water and it is night and day. RO systems are not that expensive.
It does kind of spoil you. Every once in a while I'll have a fountain soda and taste the chlorine.