I guess it's preference more than anything.
For myself, I say use insulbrite.
The reason why is because I made a ironing board for classes and workshops. I took an 18" x 24" board covered it with a layer of insulbrite, batting and canvas and in that order. and it's better than any traditional ironing board I've ever worked on especially for items that need a crisp, rigid edge.
As for the Ironing Board I made - The insulbrite and batting were exact in size to the top of the board. And the canvas was 3" more (21" x 27") around all the edges. The corners of the board I sanded, then place the insulbrite, then batting and topped with the canvas. I pulled the canvas as tight as I could and stapled it to the back of the board. Once it was all stapled I took a 18" x 24" piece of felt to cover the staples on the bottom and glued it on, covering all the staples in order to protect the surface you will place the board on. I know it's alot of information but it's a handy tool and others may be interested in making one of their own.