Just to expand on Jack's point a bit, because it is an important one:

COALESCE( expression [ ,...n ] ) returns the data type of the expression with the highest data type precedence.

ISNULL(check_expression, replacement_value) returns the same type as check_expression.

So:

SELECT ISNULL(CAST(NULL AS INT), 5.5) -- Returns 5

SELECT COALESCE(CAST(NULL AS INT), 5.5) -- Returns 5.5

SELECT DATALENGTH(ISNULL(CAST(NULL AS VARCHAR(5)), N'Hello')) -- Returns 5

SELECT DATALENGTH(COALESCE(CAST(NULL AS VARCHAR(5)), N'Hello')) -- Returns 10