As Jeff says, it depends on the scale.

But if you are talking about a small business startup I'd say use AWS and one of the RDS options as a starting point.

The other thing is to look at your requirements. If you suddenly find that you need an RDBMS and capabilities that look like SSRS, SSAS and SSIS then I'd look at SQL Server. If you only need a small part of the SQL Server eco system then perhaps not.

I'd reitterate that the decision has nothing to do with the quality or lack of features within SQL Server. It is purely on getting an acceptable solution at an acceptable cost to start and grow the business. Remember that a high proportion of business startups fail in the first 18 months.