Optimize insulation cost / benefit
Where are you located and is this for a 4-season living space?  

You want to insulate your ceiling to keep the warmth in (if in a climate with a heating season) or keep the heat out (if in a climate with a longer cooling season).  Assuming you are in a region where you need to heat your house.

You will lose more heat through an uninsulated ceiling than you could ever recover through better wall insulation or better windows.

Make sure you extend your insulation over the exterior walls to the edge, otherwise you will lose a lot of heat through the corners at the exterior wall.