They're the same thing (in API Level 8+). Use match_parent.

FILLPARENT (renamed MATCHPARENT in API Level 8 and higher), which means that the view wants to be as big as its parent (minus padding)

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fill_parent: The view should be as big as its parent (minus padding). This constant is deprecated starting from API Level 8 and is replaced by match_parent.

http://developer.android.com/reference/android/view/ViewGroup.LayoutParams.html