Since they are both on the unit circle, a and b differ only in angle. Without loss of generality we can set the angle of b to be zero, so that b = 1.
Then \(|\frac{a-b}{1-\bar{a} b}|=|\frac{a-1}{1-\bar{a}}|\)
If we let \(a=e^{i\theta}\) then \(|\frac{a-1}{1-\bar{a}}|=|\frac{e^{i\theta}-1}{1-e^{-i\theta}}|=|e^{i\theta}|=1\)
So, as long as \(\theta \ne 0\) the ratio is 1