Xamarin Forms Styling using StyleClass vs Style attribute

We're building a Xamarin Forms app we've noticed we could style an element in 2 ways by creating styles in the App.xaml ResourceDictionary

Class and StyleClass option

In App.xaml we'll write

            <Style Class="EntryStandard" TargetType="Entry">
                <Setter Property="TextColor" Value="#575e62" />
                <Setter Property="BackgroundColor" Value="#9facb3" />
                <Setter Property="FontSize" Value="14" />
            </Style>

Then this gets used in one of the contentpages like this

    <Entry StyleClass="EntryStandard" Placeholder="Login Name" Text="{Binding EntryEmailAddress}" />

Key and Style option

This is what we write under App.xaml

          <Style x:Key="ButtonMainMenu_Purple" TargetType="Button">
                <Setter Property="BackgroundColor" Value="#5d4785" />
                <Setter Property="FontSize" Value="14" />
                <Setter Property="TextColor" Value="#FFFFFF" />
            </Style>

And then we use the following in our contentpages

<Button Style="{StaticResource ButtonMainMenu_Purple}" Text="Friends" Command="{Binding OnFriendsButtonCommand}" />

Both work fine, I just wanted to know which one is better than the other and why?