Ask HN: Open source vs. proprietary technology for our project?

Wrong question. Build it in what you can build it fastest in. You need concept validation before anything else, scale and cost don't matter if the idea fails to get traction.

And incidentally, there is open source asp.net, Mono, which you can run on linux.

Two things:

a) PHP has multiple platform support (ie. Windows, OSX, FreeBSD, Linux) so you can more easily transition when necessary and

b) the OS licenses required when you scale out horizontally are cheap (free!) for Linux, FreeBSD, which are the de-facto PHP platforms. (ie. think multiple database servers with multiple web servers with multiple load balancers, etc.)

Assuming you see yourself succeeding with your projects, always think 2 or 3 years down the road to what your system uptime & maintenance costs are going to be.

As an aside - when it comes to hiring developers, I'm guessing that PHP developers are cheaper and more plentiful as well.

Whichever one you have to Google the least is the best solution. Proprietary-or-not at the web server level is not even worth thinking about - the only part of MS you have to be cautious about is if you accidentally an SQL Server farm and that's easy to avoid in this day and age.

If you go with .NET then make sure you check out BizSpark, you can get free Visual Studio, Windows and everything else they make for 3 years:

http://www.microsoft.com/bizspark

Whatever you do, avoid licensing costs. Licensing is Microsoft's core business, so...