I agree with wrholt.
If you say "estás guapa", it means you're making a compliment on how the woman looks at a certain moment. Same for "te ves bien/guapa/bonita..."
All of these express a compliment for a moment in time.
If you say "eres guapa/bonita/linda...", you're making a compliment on the permanent features of the woman.
Most women I know, however, if you say "estás guapa", "qué bien te ves" or so, would take it as a compliment and be glad that something made them look better at that moment.
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