What is the difference between Solr Replication and Solr Cloud?

I'm supporting on rails project, which contain rails app and additional instance with Solr.

My environment: rails 3.2.1, ruby 2.1.2, sunspot 2.1.0, Solr 4.1.6.

Problem:

Clould provider is not stable. I cannot use other cloud provider - it is customer's demand. Oftentimes search server goes on maintenance and web application stop working on production. So, I think about how to make 2 identical search servers instead of one, to make system more stable: if one server will be down, other will continue working.

Way out of trouble:

• or configure two search servers as Solr Replication
• or configure two search servers as SolrCloud

Question:

1. What to do and why?
2. Is SolrCloud is better solution?
3. What is PROC and CONS of these variants?
4. In which cases is better to coose SolrCloud?
5. In which cases is better to coose Solr Replication?