m-n=(x^2-8x+20)-(-x^2+4x-3)
=x^2-8x+20+x^2-4x+3
=2x^2-12x+23
=2(x^2-6x)+23
=2(x^2-6x+9)-18+23
=2(x-3)^2+5
≥5>0
∴m>n