Basically the difference between a Template and ClientTemplate is where it is executed. As the name suggests, the client template is rendered on the client-side and uses Kendo UI template syntax. In contrast, the FooterTemplate is executed on the server and uses Razor (or ASPX) syntax. Since the Grid uses Ajax instead of Server binding I would recommend using client templates.
Regarding your second question - the Sum aggregate will concatenate the values in case they are not of numeric type, and this is the expected behavior.
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