What is better in WPF for UI layout, using one Grid, or nested Grids

I am making a UI in WPF, I have a bunch of functional areas and I use a Grid to organize it.

Now the Grid that I want is not uniform, as in, some functional area will span multiple cells in the Grid. I was wondering what the best practise is in solving this. Should I create one grid and then for each functional area set it to span multiple cells, or should I split it up into multiple nested Grids.

In this image, the leftmost panel (panels separated by the gray bar) is what I want. The middle panel shows one grid where the blue lines are overlapped by a functional area. The rightmost panel shows how I could do it with nested grids. You can see the green grid has one horizontal split. In the bottom cell is the yellow Grid with a vertical split. In side the left cell is the red Grid with again a horizontal split. Grids http://www.freeimagehosting.net/uploads/08f2711bae.jpg

I was just wondering what is best practise, the middle or the right panel.

UPDATE: Just for clarification, a more 'code oriented' example:

The Middle panel

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="25" />
        <RowDefinition Height="*" />
        <RowDefinition Height="*" />
    </Grid.RowDefinitions> 

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="200" />
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>

    <Menu          Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" />
    <uc:Info       Grid.Row="1" Grid.Column="0" />
    <uc:Control    Grid.Row="2" Grid.Column="0" />
    <uc:Simulation Grid.Row="1" Grid.Column="1" Grid.RowSpan="2" />
</Grid>

The Right panel:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="25" />
        <RowDefinition Height="*" /&g