I would be wary to join a program like bizspark only because I don't think that MS software is really a strategic direction that statups should take. ESPECIALLY, technology startups. While BizSpark is a great way to set up development environments, you're still responsible for licensing fees on hosting your software, or any software from Microsoft you put out into the field. These costs can add up - especially if your software is an on-premises offering.
Additionally, there is more open source software on other technology platforms (Java for example) than there is on .NET, so there is less reason to re-invent the wheel.
Moreover, other than Microsoft software, traditional hosted software stacks like PostgreSQL/MySQL for DB, Apache/NGINX for Web, Ruby/PHP/JSP/GSP/etc for delivery run better on Linux than they do on Windows.
My experience tells me that MS software also has more operational overhead and thus utilizes your server less efficiently than comparable software on Linux.
So, unless you're doing all .NET and all Windows, I'd stay away from BizSpark. However, if you plan on doing it all in MS' walled garden, BizSpark is cheap and easy.
That said, if you want to get something up and running quick, don't have much Linux expertise and want to just put a prototype in place, Free is a good price. Just make sure you understand the application requirements before proceeding into .NET in any strategic way (long term, in other words).