Clustering and replication achieve two different things. Clustering is a high availability solution while replication is for disaster recovery scenarios. Think of clustering like having a fully redundant server (like RAID hard drives). All the hardware needs to be exactly matched. Running clusters with mismatched hardware is unsupported. The licensing is much more expensive (you need 2 copies of everything) and requires shared storage. Shared storage means you need something called a SAN. (iSCSI on a NAS also works too though). Also expensive.
There is an enormous difference in cost and they do very different things. Clustering is usually reserved for very large clinics.
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