Quite simply, the sensation you get when accelerating (pushing into seat) is a function of torque AT THE WHEELS. An 8 speed box will have close ratios compared to the 6MT, so is likely to have a higher torque amplification in each gear (due to the higher ratios), same way albeit to a lesser extent as the fact that 1st gear feels stronger than 3rd gear. This means that the average torque at the wheels will be higher with the 8 speed box, therefore feel faster......and is faster.
Technically, if you had a common gear ratio between the two cars (assuming the diff ratio is the same too), and did a roll on race in one gear, the MT should be marginally faster....a.) because it is lighter both physically and inertia wise b.) because it uses more efficient (simple) gear systems c.) It doesn't use engine power to drive an oil pump.....etc.
What you prefer is an entirely different subject.
