Andarine S4 is mostly a strength oriented SARM, so you will get a good boost in strength with barely any lean mass. Therefore, between the two, Ostarine is the best bulker. However, there is also another SARM to keep in mind for bulking - Anabolicum LGD-4033. In any case, I would recommend Ostarine for a better improvement in recovery, but if it would be possible, running both SARMs would definitely give a more pronounced growth.