Should I use an iron angle or an iron plate to support brick above window?
A 3/8" plate may be a little too thick to fit without fattening up the brick joint. A 3 1/2"X 3 1/2"X 1/4" is a little overkill but would keep from creating a fatter joint. With all the brick I laid, for that size window that is what has ever been used. Although structurally the 3/8" plate would work too, I would still use the angle. Otherwise you may notice the joint grow in the area over the window. Also, as a side note, don't let any mason tell you he can't match the color of the existing mortar because of sand color or other BS like that. Sand does play in the color of the mortar, You can pick out whether it is orange or white or yellow with a close look, but the mortar mix itself is the main influence here for color. They are available custom color at masonry supply houses for just a few bucks more a bag. If the mason uses a "just add water" type mix, it WILL be gray... really gray. For 10 bucks in sand and 20 bucks for a bag of custom color if need be, it will cure the distraction.