You should use a Grid if you need things to line up horizontally and vertically. Use a StackPanel to create a row or column of things when those things don't need to line up with anything else.
However, don't limit yourself to those two options. In particular, have a look at the DockPanel. It's slightly more complex than a StackPanel, but its markup isn't as cluttered as the Grid. Here's a good article on the DockPanel:
Using the DockPanel in Silverlight 2
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