Some gear does require a 2 way connection since it both sends and receives…an MPC falls into that category since it’s a sequencer (sends/receives), a controller (sends) and a sampler (receives). But the SP is just a sampler…it only receives, which is why it only has a MIDI input. That’s all you need to sync it to your computer.
The fact that the SP can’t be used as a controller (because of the lack of MIDI out) is a valid point, but it definitely doesn’t make it useless. Just means that, in a sequencing environment, it would have to be controlled by something else.
But comparing these 2 is ridiculous since they serve entirely different purposes. Like comparing a Harley to a canoe. Judge each one on its own merits (not on how they compare to each other) and you’ll see they’re both great products…as a matter of fact they complement each other. Which one you should choose depends on what you want/need to use it for.