Sashing is better - much more pleasant for the eye with a T-shirt quilt. Since it will be hung, orient them the same. But, as far as the layout, you have to lay them out and adjust. Only then will the colors make sense and balance. If you don't have a design wall, lay them out on the bed. I would use a sheet on the bed of a similar color to what your sashing will be. Then leave the room and re-enter several times to see if one area looks out of balance. I like to use a sheet because I can roll it up and carry it to my workroom.