BDO who assists DM and to become a DM through state PSC, then it becomes a difficult tasks. Although a BDO officer or executive service officer can also be an IAS office but becoming a DM is difficult because suppose you crack state PSC at the age of 22 and with luck and your best services one day you'll become an IAS officer but your highest post will be a SDM post not a DM. So become a DM, clearing UPSC exam is recommended.