[nelhage]

They each have different advantages and strengths.

MongoDB is great for failover and for rapid development or prototyping. SQL is great for reporting or analytics, since you can do all kinds of aggregates and JOINs right in the database.

The edge cases where you can't represent the data perfectly aren't a huge deal for this use case -- because it's a one-way export, you don't have to be able to round-trip the data, and as long as you can export the data you want to run analysis on, it doesn't matter if there's some you can't get.