Do them both on 24"
How are you mounting? If mat and glass the mat covers the edge. If going in a frame does't matter again as the frame rabbet will cover the edge.
Just printed 25 canvas 24x36 that were mounted to gatorboard and framed with metal frames. Printed them all on 24" canvas with a 1/8" white border on the sides easily covered by the frame. To keep from loading up yours pads with ink I would do the same thing.