Espalier is a technique of pruning and training apples in two dimensions, usually against a flat surface such as a wall or fence. The rootstock that you choose needs to induce enough vigor in the scion cultivar so that growth occurs but does not produce a tree with too much vigor. The environment where you are located will alter this decision. The colder, northern areas of the United States — that is, USDA hardiness zones 3 and 4 (look up your hardiness zone at http://www.garden.org/zipzone/) — would need a more vigorous rootstock such as M.26 EMLA to induce growth, whereas hardiness zones 5 or 6 would need a more dwarfing rootstock such as M.9 EMLA. The scion cultivar is of utmost importance in this system, with spur-bearing cultivars recommended over tip-bearing cultivars. Spur-bearing cultivars form fruit on very short branches called spurs. Many apple cultivars bear fruit on spurs.
Answer provided by Emily Hoover, University of Minnesota.