Vladimir Oselsky notes an edge case where ISNULL and COALESCE can behave differently:
Even though we would expect to see both records returned we only get 1 record. Huh? This is exactly what puzzled a coworker, ofcourse query was not as simple as this one but same issue caused him to hit a road block.
In the case of COALESCE and OR methods, results are identical.
The underlying issue here is that the variable data type differs from the column’s data type, and exposes a difference in how COALESCE and ISNULL work.