Hmm, really light winds. I'd say the Quad 77. I don't think going Quad 82 will gain you anything with a 5.0. If you're used to schlogging on small boards even Quad 74 will work (I was out in the conditions you describe on the 74 last week, ie up to head high and 5.0 ranging from no power to fully planing, I had the 82 in my car but was really happy on the 74....I'm 72kg).
If you prefer some extra volume you could of course also go for the 82. No worries sailing it in 18 knots.