If you can afford for your website to go down if need be, ie...hardware/software issues, and you want to save money then single would work.
If you CANNOT afford for your site to go down for whatever reason and can and are willing to spend the extra money then you might want to go with a cluster.
Difference:
Single is one server, if it crashes, so does your website, until someone fixes it.
Cluster is more than 1 server, and what it does is you have a primary server (Master) and then secondary servers (Slave), and when the secondary server senses something failing on the primary it takes over and therefore your website nevers goes down.