What governs the decision to use (in SharePoint 2013)?:
server object model (ssom)
client object model (csom)
It depends on the following factors:
Server side object model : If your code runs on same machine as your Sharepoint farm. You should use Server side object model. The assemblies that define the built-in server-side classes are installed to the global assembly cache of each server when SharePoint 2013 is installed. When you program against the server object model, your assemblies are installed as farm solutions to the global assembly cache.
Client Object model: If you are accessing sharepoint data from any machine other than sharepoint server, you can use .Net managed client object model.