This will depend on the film and developer used, but I think with 9,5x12" you will see a clear difference at first glance even with films like Delta 100. Maybe already at 8x10" if you use Hp5+ or Delta 3200.
With good negatives from both formats and good printing technique, the difference is very apparent at 8x10 inches. Films like Techpan and Panatomic-X were the best for smooth tones.
The difference in quality between 35mm and 6x7/6x9 is very pronounced, more so than the difference between MF and 4x5.