If you intend on having 2 servers replicated, then you can use Master-Master replication. This means that either one of the database servers can go down without loss of data or access. This is extremely resilient to failure however you can get duplicate key errors on fast successive inserts, like using MySQL to handle sessions. This can be solved programatically through.
The downside of the Master-Slave set up is that if the master fails you have to manually assign another Master, fix the failed master and then bring back into the group. Otherwise failure of any or all Slaves will not affect the Master.