I used to sell Luxman gear back in the late 1970s, and as I recall the U model was the standard export version of the PD-121 w/ a universal (switchable) power supply. I do not remember ever seeing or selling the PD-121A model, but I have met guys who have claimed to own them, so my mention is more allegorical than real.
Also, the keep in mind that the PD-131 is also a good deck; it was originally offered as an armless version of the PD-121.
In comparison to the SL-1200 I can only offer you my opinion, others may certainly disagree. I believe that the PD-121/131 sounds a little more musical and lush and offers much better bass control than the Technics turntable. But this is a tough comparison, as you will never see these units rigged in a comparable way. As I have mentioned, one of these decks w/ a good, midpriced tonearm and matching cartridge will almost certainly cost more than an SL-1200 but provide a great sound/price ratio.