you haven't really said how you are replicating the data, and there are so many replication solutions out there....
In general, my view is that with streaming replication you really want your slaves to be identical to the masters in as many was as possible. Failing back is not a simple process and it requires restoring, effectively, the primary with a backup made from the slave. For this reason it is good to plan on having an ability to be without your preferred master for a while or even be able to fail back and forth with neither node being preferred in that role.
Your best bet is to have these to be identical and to scale reads by adding more slaves.