In my opinion a tax system which causes equal agony to everyone is the fairest. Then everyone has an incentive to see government be as frugal as possible with the tax dollar. Of the two choices I pick flat tax, although many of the flat tax proposals have very high deductables which defeat the "equal agony" concept.
I agree with you on the equal agony concept. This is the best way to maintain more limited government, because people are much more likely to favor destructive expansion of government when they think somebody else is largely paying for it.
I think though that to achieve equal agony, a progressive system is more appropriate. If tax rates are flat, the relative level of agony is much higher for those with lower incomes than for those with higher incomes.