Quick takeaway
Panasonic NN-SD975S Cyclonic and Panasonic NN-SN686S 1.2 Cu Ft are very close in this matchup. When the overall score is tied, the right choice usually comes down to price, review depth, and which spec wins matter most for your use case.
Choose Panasonic NN-SD975S Cyclonic if...
Pick Panasonic NN-SD975S Cyclonic if power and model number are the reasons you are shopping in this category.
Choose Panasonic NN-SN686S 1.2 Cu Ft if...
Pick Panasonic NN-SN686S 1.2 Cu Ft if its current price, brand, or design fits your shortlist better than Panasonic NN-SD975S Cyclonic.