Newsletter SQL query optimization
Use EXPLAIN in the sql console to determine what the query optimizer is doing with each query.  This will give you clues as to how each query performs, and may also help you optimize further, for example by indicating places where adding an index will speed the query up. 
Run each of the queries in a loop and benchmark how long they take! If you're using PHP just wrap the query execution in a for loop and execute it 100 times or so.  Record the microtime() before you start and do it again at the end.  Subtract the start time from the end time and you'll have your benchmark.