I have been putting together a list of reasons why Alighten focuses primarily on ASP.NET instead of Java/PHP and other languages. Then I stumbled on to this interesting post by a startup company and why they chose to go with ASP.NET. It summarizes most of the key issues faced by today's website project managers, website programmers, and website designers.

http://blog.jumptree.com/2007/why-we-chose-aspnet-to-run-our-startup/

It's important for you to discuss the pros and cons of each technology when discussing them with your clients. I hope this post provides you with a good starting point to do just that.