Here's a tip. Proc Append is particularly useful when appending a small dataset to a large dataset. If you use the example above using the SET statement, every row from each of the datasets is read and processed. With Proc Append, only rows from the second (data=) dataset are read. So if you have one large dataset (such as a historic file) to which you are appending a smaller dataset (current weeks data for example) then Proc Append is a more efficient way of coding it. Otherwise there really isn't much difference between the 2 methods.
Another alternative method is to use the UPDATE statement instead of SET and this will update records on the master dataset with the records from the transaction dataset with matching BY values. Probably not what you need in this case though.
Enjoy.