Equal Voucherization?

Rick, I'd be happy to move to "universal ... voucher-ization of education funding, with government schools being one co-equal option among many other (reasonably regulated) options."  But why should that voucher-ization be "equal" among different income levels, and not just (as I agree it should be) equal among different educational options?  Why not still recognize that the wealthy have a greater capacity to contribute to their children's education, and that low- and modest-income families will have a limited choice of schools beyond the voucher amount (unless you require all schools to accept the voucher as full payment), and therefore that we could still achieve greater choice in real-world terms by making the size of the voucher greater for lower income levels?  I grant that in a fully voucherized system with no free public-school baseline, you'd need more in voucher aid for high-income levels, since the voucher is replacing the free public school.  But still the graduated voucher seems to me more attuned both to the economic realities of choice and to the emphasis on empowering the poor.  I guess another way of saying this is that the current system of simply giving everyone a free education in public schools, regardless of family income, is non-progressive in its structure, as well as non-pluralistic in its favoritism of government schools.  Why not seek more progressivity in the benefit structure as well as pluralism in options, as a matter of principle (even though this approach is probably even less politically viable than the others we've been discussing)?

Tom