SelectedValue vs SelectedItem.Value of DropDownList

They are both different. SelectedValue property gives you the actual value of the item in selection whereas SelectedItem.Text gives you the display text. For example: you drop down may have an itme like

<asp:ListItem Text="German" Value="de"></asp:ListItem>

So, in this case SelectedValue would be de and SelectedItem.Text would give 'German'

EDIT:

In that case, they aare both same ... Cause SelectedValue will give you the value stored for current selected item in your dropdown and SelectedItem.Value will be Value of the currently selected item.

So they both would give you the same result.

SelectedValue returns the same value as SelectedItem.Value.

SelectedItem.Value and SelectedItem.Text might have different values and the performance is not a factor here, only the meanings of these properties matters.

<asp:DropDownList runat="server" ID="ddlUserTypes">
    <asp:ListItem Text="Admins" Value="1" Selected="true" />
    <asp:ListItem Text="Users" Value="2"/>
</asp:DropDownList>

Here, ddlUserTypes.SelectedItem.Value == ddlUserTypes.SelectedValue and both would return the value "1".

ddlUserTypes.SelectedItem.Text would return "Admins", which is different from ddlUserTypes.SelectedValue

edit

under the hood, SelectedValue looks like this

public virtual string SelectedValue
{
    get
    {
        int selectedIndex = this.SelectedIndex;
        if (selectedIndex >= 0)
        {
            return this.Items[selectedIndex].Value;
        }
        return string.Empty;
    }
}

and SelectedItem looks like this:

public virtual ListItem SelectedItem
{
    get
    {
        int selectedIndex = this.SelectedIndex;
        if (selectedIndex >= 0)
        {
            return this.Items[selectedIndex];
        }
        return null;
    }
}

One major difference between these two properties is that the SelectedValue has a setter also, since SelectedItem doesn't. The getter of SelectedValue is faster when writing code, and the problem of execution performance has no real reason to be discussed. Also a big advantage of SelectedValue is when using Binding expressions.

edit data binding scenario (you can't use SelectedItem.Value)

<asp:Repeater runat="server">
 <ItemTemplate>
     <asp:DropDownList ID="ddlCategories" runat="server" 
                       SelectedValue='<%# Eval("CategoryId")%>'>
     </asp:DropDownList>
 </ItemTemplate>
</asp:Repeater>

One important distinction between the two (which is visible in the Reflected code) is that SelectedValue will return an empty string if a nothing is selected, whereas SelectedItem.Value will throw a NullReference exception.