At least for prototyping & early development, I'd say you want the schema flexibility & rapid development that a relational database can give you. SQL lets you

At least for prototyping & early development, I'd say you want the schema flexibility & rapid development that a relational database can give you. SQL lets you issue very complex queries incredibly easily (both for production & for measuring your own metrics). With alternative systems you'll have to do a lot more work to get the same results; I'd say SQL is invaluable for getting something working. Once you've locked down your schema & have a good idea of the queries that run, then you're in a much better position to evaluate the various technologies. If you choose an 'alternative' DB, it'll be fairly easy to get your data out of MySQL, whereas the converse often doesn't hold. Finally, it seems to be the fashion to underestimate how much data a relational database can handle, while taking it on faith that alternative databases can handle a lot more. Once you have real data, you can do your own comparison, rather than just buying into the hype.