@if (!Model.Id.Equals(null))
        {
        @Html.DropDownListFor(m => m.Modificar.IdClase, new SelectList(Model.ListaClases, "IdClase", "Clase", Model.Modificar.IdClase), new { @class = "input form-control input-lg", @required = "required", @style = "height: 40px; font-size:16px" })
        }
        else
        {
        @Html.DropDownListFor(m => m.ListaClases, new SelectList(Model.ListaClases, "IdClase", "Clase"), new { @class = "input form-control input-lg", @required = "required", @style = "height: 40px; font-size:16px" })
        }

var clases = $('#Id').val() != "" ? $('#Modificar_IdClase') : $('#ListaClases');