ASP.NET MVC: Razor @helper vs extension methods to HtmlHelper - which is preferred?

Subjective question, so here's my subjective and biased answer: When the helper code involves amounts of C# code use a custom HtmlHelper and when it's primary markup you could use @helper. But assuming that when you have markup you could use a partial like @Html.Partial("_foo", SomeModel) or an editor/display templates like @Html.EditorFor(x => x.Foo), the @helper doesn't really have any practical use. Personally I've never used @helper by the way, and I've never recommended it's usage to people I've been consulting.