You would save $900 by choosing UA. You get 125 or 150% miles ( have to check Aeroplan )
You get business class service instead of fancy tourist service ( rouge is billed as being low cost for tourists, do you really want to be the top of the heap in a plane load of tourists?)
And you get to stick it to AC for degrading themselves and their customers with this rouge foolishness. After your flight you should email Cailin your rouge itinerary and your purchased UA itinerary to rub his nose in it.