The ducting is never an issue, maybe he is American so they have fewer 5 inch ducting options but you are right, 5 inch is everywhere here. I cant understand though why you are making this such a big decision for yourself, everyone is saying go bigger on here, the bigger fans sucks more, it's only a tenner more and the extra price on fittings is minimal, why are you stalling so much on this??
Surely if you are uncertain then you should express your reasons for this, otherwise you will have people pooping 6 inch fans all over this thread till the cows come home, buck up and make a decision man!!!!!!!!!!!!!!!!