I have an engineering sample of the U9-S, and I've used the Das.
- Looks nicer
- Tighter tolerances
- Better keycaps
- USB/AV passthrough
- More expensive
- Requires constant polishing
- Cheap (MSRP is going to be in the $80 range)
- Mediocre keycaps
I have my step-dad using the U9-S ES. About the same as the retail model, except with the top housing half from a U9BL, and no NKRO (since it's a single layer PCB; retail model should have NKRO). It's pretty much the basic, barebones mechanical keyboard with MX browns, no frills or anything out of the ordinary, and acceptable build quality. Only real thing I can charge against it is that the laser process used on the keycaps isn't very good, you can feel the lettering very distinctly and there are reports it wears quickly.
Das keycaps also wear quickly for some reason, but feel much smoother, and there's no wear at all if you get the blank version. It looks nicer, but it won't stay that way if you don't polish it daily. The USB passthrough is convenient occasionally, but only occasionally. The Das feels a bit more solid due to tighter construction tolerances and a thicker steel plate.
Otherwise they're very comparable and the biggest difference is price.