I'd definitely vote for PostgreSQL, it can handle large loads effortlessly, it's reliable, and yet they keep adding great features.
It could also function pretty much like a nosql db initially, to ease your transition, then you could migrate gradually to using it as a relational db. You need strong checks on data integrity before you start - you could consider double writing (to old orm using nosql + new orm using psql), and comparing data stored to be sure you don't miss anything at first, before you switch?
It could also function pretty much like a nosql db initially, to ease your transition, then you could migrate gradually to using it as a relational db. You need strong checks on data integrity before you start - you could consider double writing (to old orm using nosql + new orm using psql), and comparing data stored to be sure you don't miss anything at first, before you switch?