It's true that the resolution difference between 720p and 1080p will be indistinguishable from that distance. Another point to consider, however, would be scaling issues. 1080i material will just get de-interlaced, therefore, no scaling artifacts. 1080i and1080p sources will have to be converted to 720p through scaling which may cause artifacts in the video under certain conditions. That's just my speculation, by the way, so don't consider that as more than a single data point.
My feeling is buy the best you can afford. If the S1 is out of reach, get the X1 and don't sweat it. If the extra cash for the S1 isn't too big of a burden, I'd go that way and not have to worry about buyer's remorse.
Jim