Depends on how hardcore you are. If you require the very best experience possible, assuming you can afford it and are willing to pay for it to have that experience, then you probably want to get the XBSX.
If you're not that hardcore and just want something that plays next gen games reasonably well, with limited BC capabilities, etc, or simply can't afford it, then XBSS will be just fine, for a while anyway.
If you buy the XBSS, you need to do so assuming you may need to upgrade to XBSX 3 or 4 years in for your gaming experience to remain the same. You may not have to, but you need to be prepared to. By that time, XBSX should be $299-$399 and may possibly be all digital as well.
XBSS will be able to operate like XB1S does now. It'll play games at 900p (or lower) if it has to and upscale them to 1080p, likely later in the gen.
Truth is you won't really know how this will play out down the road. Will MS drop an upgrade in 3 or 4 years, or will they have a shorter gen and come out with a 4k and 8k console for next gen? Nobody knows for sure.
