I have both. Currently working on a review. So far they sound very good (and very similar to each other). I think the cheaper $100 one (forgot which number goes with which) is at least as good a value as the RE-400 from HiFiMan.
Two small nitpicks so far: I am not a big fan of the stock silicone tips and the housings are a little on the large side (13.5mm drivers and all that). Once sealed properly they definitely deliver, though.
So would you say the S2 (more expensive) is worth the extra over the S1, since you mentioned they sound very similar.