For wizards (and likely WDs), keeping the % reduction similar is nearly impossible, or impractical at best. Even with our AR increased by our high Int, AR rolls are almost always better than Armor rolls for most wizards (exceptions tend to include those slots that roll 500-590 armor and depend a little on skill choice, like prismatic armor vs pinpoint or crystallize).
In the end, go for whatever stats give you more reduction and don't worry too much about the whole 1:1 ratio of damage reduction (or 1:10 AR to armor ratio people like to throw around).
On a side note, the overall calculation is simply total DR = 1 - product(1-DR_i) for i different damage reductions. So for just Armor and AR, it's 1-(1-DR_ar)*(1-DR_armor)
In the end, go for whatever stats give you more reduction and don't worry too much about the whole 1:1 ratio of damage reduction (or 1:10 AR to armor ratio people like to throw around).
On a side note, the overall calculation is simply total DR = 1 - product(1-DR_i) for i different damage reductions. So for just Armor and AR, it's 1-(1-DR_ar)*(1-DR_armor)
Crusader DPS and EHP Spreadsheet, meant for Crusaders
My Wizard