... the public folder store in the same storage group as mailbox store. The public store is its own distinct database in the storage group, PUB1 versus PRIV1, the mailbox store.
You also may want to replicate folders to other server(s). Generally, if you put replicas of folders on the same servers that you anticipate most of the folder access would be from, you can minimize the amount that exchange may have to contact a different server to grab the contents of a folder to service a user request.
For example, if all of the sales force mailboxes are homed on one particular exchange server, you want replicas of the public folders that they are most likely to use, on the same server.
The converse, if you replicate everything everywhere, then you give up performance to replication traffic in trying to maintain too many replicas.