S chips are low power versions. For 99% of people the non s versions are the way to go (no letter is better...except k which is unlocked multiplier). S are generally slower as well (the 2405s runs at 2.5 ghz vice 3.0ghz for the 2320). The only advantage of the 2405s is a better integrated graphics (not a big deal).
Summary: ignore s models IMO.
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