Not much of one, unless you want a wider front. Spears get the rank bonus for 2 ranks behind the front, while pikes get 4 ranks. Therefore, pikes are optimal at 5+ ranks, while spears are optimal at 3+, so it's not best to stretch your pike formations out. But since they are the same unit size, the formation becomes square. Because of this compact formation, pikes can get flanked easier if you are not careful with your lines.