Your Gabriel Knight 3 example is a simple case of breaking the fourth wall, which I don't mind at all, in fact in can be rather fun (didn't Rincewind ever tell you to stop clicking him?). It can probably be a bit weird when it happens in an otherwise serious game, but if the game has some humour in it, then by all means, destroy that fourth wall.
Otherwise I prefer your b) variation where the character just says he's not going to do something and then states the reason.
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