Well honestly if you use the 4-6 range but do most of your sets with 6 reps or if you use the 6-8 rep range and also do most of your sets for 6 it will obviously be the same Even if you do most of your sets for 7 reps it will be a lot alike.
The 4-6 and 6-8 zones aren't so much about reps as they are about load.
Most people will be able to use between 82.5-87.5% when using the 4-6 range
Most people will be able to use between 77.5 to 82.5% when using the 6-8 range
The type of training effect is determined by the load, not the number of reps. The number of reps done in the selected rep range will affect more the magnitude of the gains
So if you want to use the 4-6 range to get the increase in hardness with some muscle gain you should use 85% of your maximum using the double progression model. Using 87.5% might be too much because you would not be able to get 4 reps in the 3rd+ sets.
If you want to use the 6-8 range you should use 80% and also use the double progression model. Using 82.5% would be too much because even at the 2nd set you are likely not to hit 6 reps.
Now are the effect THAT different between 4 to 6 and 6 to 8? In the grand scheme of things it will not be big. Honeslty it is already VERY hard to add even a small amount of muscle than even a big difference between two methods would not lead to a huge visual difference.
The hardness effect might be a bit more noticeable, but only if you are lean. A musclke wont look hard regardless of the method used if it is covered by 1" of fat