I would choose MySQL/PHP for the following (mainly performance-related) reasons:
PHP may be cached, accelerated (pre-compiled) and compressed on the server by using e.g. a combination of PHP-Accelerator (http://www.php-accelerator.co.uk) and mod_gzip (http://www.schroepl.net/projekte/mod_gzip) on Apache on a UNIX-flavor. You may choose a commercial product if you like - Zend is known to work very well (http://www.zend.com/store/products/z...ance-suite.php) but PHP-Accelerator and mod_gzip is offering nearly the same performance for free, although without the same level of customer support.
MySQL is blazingly fast in version 4 if you activate the new cache that allows not only SQL-queries but also the result from the query to be stored in a cache. It requires some additional memory but the performance increase is simply amazing. Note that version 4 also includes transactions in a default setup - this is a much sought after feature that took some tinkering to activate in 3.23.
eWEEK did a comparison of several Enterprise-grade databases (including MySQL 4.0.1) here:
And yes - MySQL 4 is still in beta but in my experience it is just as stable as 3.23 when used for the same type of work, even under very heavy load.
TCO on ASP/MSSQL and PHP/MySQL? Well, I really don't think there is going to be that much of a difference when it comes to maintaining the servers but you might be in for a nasty surprise if you need to upgrade or expand your initial setup:
While adding another server with a MySQL-slave is going to cost you the hardware for the server, another MSSQL-server is going to cost you substantially more just for the software. Even adding a second CPU to your database server is going to cost you since MSSQL is licenced per CPU, not per server.
If you (or your developers) are most comfortable with ASP you should probably stay with that to keep development cost down but by all means go with a MySQL on a UNIX-platform instead of MSSQL - both for performance and price.
Anders C. Madsen
Golden Planet Support - http://www.goldenplanet.com