If you really want to get good results, you can install Linux or get a hold of a Linux server or a Win32 version of AB (ApacheBenchmark), and do something like
ab -n 20000 -c 20 http://www.mysite.com/script1.php
then compare with
ab -n 20000 -c 20 http://www.mysite.com/script2.php
where 20000 is the number of requests and 20 is the number of simultaneous requests.
Measuring time by microtime is good, but doesn't take into account the time to server/database caching, repeated access, etc. Hope this helps 🙂