I would go for PHP too but not MySQL so not LAMP but PostgreSQL.
Why PHP?
- Because it can run on many platform Linux, BSD, Solaris, Mac OS, ... and eventually on Windows.
- Until proven otherwise, there is a bigger community on PHP side, which mean more help when you will need it and a lot of code already available.
- Using Linux, Apache, PHP, PostgreSQL and smarty as templeting system that would give you a highly powerful system and all of it for free.
- PHP is GNU licensed, ASP.NET is not.
Why PostgreSQL?
For the why PostgreSQL and not MySQL you can read this and make you own judgement :
http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL
At the end of the day, that would be MY choice, but we are in a "free" world, so it might not be YOUR's.
Angel