There is only 1/8" difference in total diameter over the smaller tube. AND, when you split that on either side of a tube, from an imaginary center line down the middle of the tube, then the larger tube is only 1/16" bigger on either side. Very minimal difference. B.
If you are staying N/A, definitely get the 1 3/4" because it will help with low end torque. You lose a little low end torque to gain a little horsepower on the top with the 1 7/8" unless you're big cubes or forced induction that needs the flow. I wish ARH made the 1 3/4" because I would have rather had that but I won't buy Kooks.