The flight DPS-BNE is usually 5:25 hours. Subtract the taxi, take-off, meal service - then the wake-up one hour out, you only end up with perhaps 3.5 hours of sleep.
The 737 seats are pretty rudimentary - not long haul style extra recline or lie flat.
I think it is better value to fly J on the daytime flight - we did this early September and had a wonderful time. The 8 of us in the cabin almost drank the bar dry and had a very enjoyable and interactive time.
If you have loads of points our want to impress the SO (try to do this during your entire relationship btw) then go for it.
Otherwise, you're better to get economy X seats. Don't go for the first row (row 3) as the immovable seat dividers significantly reduce the seat width plus don't allow you to lift the divider and snuggle up together.
Row 14 exit row has the most room and presents good value.
FWIW: The best flight DPS-BNE is the 3:25pm (sometimes 3:45pm) that gets into BNE just before midnight. However, that doesn't work if you then need a connection to Sydney. Why are you not flying direct DPS-SYD.