This sounds like any old argument of computing when you have two similar software systems..
My rule fo thumb is if the client is big and has money ASP is the way to go, support is good , and although the product might be bloated and completey Windows based, it works, and it SCALES, look at all the major sites running ASP (bn.com, nasdaq.com, and of course microsoft.com, all these sites and mroe get tons of traffic), and truth be known getting a site on an NT server shouldn't be an issue for the client, if they're going to pay you big bucks to build something, the server/hosting cost for nt should be inconsequential..
For smaller more budget minded clients, who want to run everything on a shoestring budget, go with PHP, its easier to build in, (for smaller light traffic sites) and you can't beat the price...
-tony