The P1750BNLG9 uses the older, more efficient Seasonic SS-750HT / M12D design.
The P1750XXXB9 uses the Seasonic SS-750AM design, which is very similar to the designs used in the Corsair TX750 V2, NZXT Hale82, possibly a few others but can't remember them atm.
I personally would choose the P1750BNLG9, because the fan spins slower in idle (600RPM vs 1000RPM) meaning that it is quieter most of the time.
Hmm so that's another + for P1750BNLG9 in addition to being silver.
Is there any disadvantage of P1750BNLG9 since it is older design?