This was controversial for a while, but the consensus now seems to be that the R-D 1 and R-D 1s are functionally identical except for the firmware, and yes, you can download and apply a firmware update for the R-D 1 that will give it the same features.
The R-D 1s isn't even available officially in some countries, including the USA; I suspect it's simply a differently "badged" model to help distinguish between official and gray-market imports, similar to the way many SLR manufacturers give the same camera different model designations for the Japanese, USA, and European markets.
If the R-D 1s is more expensive, I'd certainly suggest buying an R-D 1 instead and simply applying the firmware update; you'll get exactly the same performance and features.
I've been very pleased with the results of the firmware update on my R-D 1; the only consistent downside people have been reporting is that the battery doesn't give quite as many shots per charge, although it's still unclear whether this is because the updated camera actually consumes more power or, perhaps that Epson simply has set a more conservative low-battery cutoff point to avoid occasionally-reported problems such as the shutter locking up or the camera failing to write data when the battery got too low.
