ISNULL() OR Is NULL in UPDATE statement
Those queries won't do the same thing.  

Update Table set REC_ID = isnull(REC_ID,'')


This one will update each record and if REC_ID is NULL it will set it to ''.  

Update Table set REC_ID = '' where REC_ID is NULL


This one will only update records containing a null value in REC_ID, and set it to ''.
While both of them will end up giving the same result, the second one will be executed on less records (except if every REC_ID is NULL), it should be faster.