@Html.Partial as HTML code copied into the parent page and @Html.RenderPartial as an .ascx user control incorporated into the parent page.
'@Html.Partial' returns a html encoded string that gets constructed inline with the parent. It accesses the parent's model.
'@Html.RenderPartial' returns the equivalent of a .ascx user control. It gets its own copy of the page's ViewDataDictionary and changes made to the RenderPartial's ViewData do not effect the parent's ViewData.