As I recall, when you take the free tour you remain in customs "quarantine"
and no formalities or airport fees are incurred. If you leave by yourself,
I suspect you may incur a departure fee when you return.

The tour will kill a few hours. But as others have stated, there are enough
[shopping/eating/movie] activies at the airport to keep you busy for 6
hours. Possibly even some free internet terminals.

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