Estimated Sales Rank is the number of sales an ASIN gets from all the combined sellers on the listing using the rank changes in a 30-day time period. This number factors in various data such as where the rank stands in the category and is based on reference points
The sales rank drops show the total number of rank drop over a certain time period and are based on Keepa's Historical data of the rank.
If you are trying to see the consistency of how well an ASIN is selling then use the sales drops. If you trying to see the velocity (how many sell) then use the estimated sales numbers.