Addition With Null Values
COALESCE can take a whole list of values to check for NULL where ISNULL takes one check value and would require nesting to check more values. 

SELECT 
COALESCE(col1, col2, col3, col4, col5, 0) AS value_returned
FROM myTable


vs

SELECT 
ISNULL(col1, ISNULL(col2, ISNULL(col3, ISNULL(col4, 0)))) AS value_returned 
FROM myTable


Since your case is using float values, there should not be a difference in results from calculations.  The only benefit I can really see if code readability and minimizing the amount of nesting needed.