BizSpark/WebSpark blow away any complaints or concerns, valid or not, of server or hosting costs as almost any valid, new/new-ish software startup should qualify.

On the other side of that, MySQL enterprise licensing is only marginally cheaper than SQL Server, and one might argue lacks features and rich toolsets to work with in the enterprise. And in the end, most web-based startups are really SaaS so yes, you really do need to be thinking in terms of enterprise, and minimizing annual cost of delivery and ownership/subscriber.