Different product segments?
The D50 is just a DAC with variable pre-outs. This is a DAC and headphone amplifier. The front port of this is variable, but designed for driving headphones, while the rear RCA can be used as either an input (if the SDAC isn't powered) in which case, it can be fed a signal from another DAC, like the D50. Or an output (if the SDAC is plugged in), in which case it will be a fixed line-out voltage for sending to another amplifier.
The D50 does have other to digital inputs though, while the SDAC is USB only.
If you're plugging into powered monitors, the D50 might be the better option, but if you're using headphones, and don't care about optical or coaxial inputs, then the SDAC is the one for you. I don't know about the D50, but the SDAC is driverless operation on most OSes
Of course, if you need DSD, or the higher sample rates that the D50 can do, then yeah. That's your man.