Webjet let's you see price comparisons

eta: I use Webjet to do the comparison then go direct to the particular airline, as others have mentioned, it is usually cheaper to go direct.
Yes this use web jet then direct to airline for actual booking.

Seating - on a Hong Kong flight you will prob get a 2 4 2 arrangement so I would go for the 4 in the middle myself. The % of time you can see out of the window is so minimal, I would go for the space of four. I also would book the cheapest irrelevant of time of day, they may sleep or may not and if is extra for a night flight x 4 is alot of money.