Yes the 6" is f/8 and the 8" is f/6. So the tube length is the same, one is just fatter but is still back-seat-friendly. Since light gathering increases with the square of the diameter, the relative light gathering is 6^2 = 36 vs 8^2 = 64. So the 8 gathers 64/36 = 1.78 times the light. Not to be sneezed at.
