Depends largely on who has to maintain the site going forward and what their preferences are. The big contenders, in my opinion, are c#/.net + SQL Server and PHP + MySQL.
C# is a strongly typed, compiled language, which I find to be pretty tedious for general web development. Though, it make SOAP service (both client and server side) development very easy. But, for simple tasks, I find that there's a lot of unnecessary overhead, but at the advantage of very rapid development. You get a lot of structures for free, which means your developer can suck and you'll still likely get a decently functioning application. But, this often causes even smart developers to default to those structures in place of more optimized, custom solutions.
Since it's compiled, the code itself is theoretically very speedy, which may make up for some of the overhead and cookie-cutter performance hits. You can't reliably produce a generalized .NET v PHP benchmark: the whole application lifecycle, request pipeline, and runtime environment are different ... different things come for "free" in each.
PHP is a loosely typed, interpreted language, but the "compiled opcodes" can be cached very easily for near-compiled-language performance. You don't get as many "full" cookie-cutter solutions with PHP, so a lot of the application-design thinking is forced onto the developer, which is a very good thing if you have a good developer writing a complex or performance critical applications. It's a pretty terrible thing if your developer isn't good.
But, you do get a very large, ever-expanding library of very webdev-useful functions. PHP provides functions and environment that spare the developer from thinking about things that simply don't need to be thought about, leaving the developer to think about things they should be thinking about, like how to structure their database, optimize their queries, and optimize their application-side code. But, if the application is simple and straightforward, you're often getting less for free. And there can sometimes be additional ambiguity in the code on account of the loose-typing.
And, as mentioned above, there's cost. PHP is cheap to develop and run. .NET is expensive to develop and run.