The answer is pretty straight forward. If your academic profile is average as I don’t know about your grades so I am here assuming that you do meet IIM-A's minimum eligibility criteria that is 10th (minimum 80) 12th (min 77 or commerce and 80 for science) and check what is your application rating under your respective graduation field. Only when you meet the minimum criteria is when you have a shot at IIM-A. Otherwise, the comparison does not even hold. Additionally, you also have to consider sectional cut offs, IIM-A would probably require 85 percentile in each section probably higher. So again you might have a crazy high percentile but may not clear certain sections. Another road block.
But when it comes to FMS faculty of management studies delhi, the road is much easier, crack the CAT exam, and later on kill it at the PI, if you manage to do both, congrats mate you are at FMS, a Top 10 college of our country, so as you mentioned that your profile is average, then FMS is easier than IIM-A.
Decision is yours, choose wisely.