Go for the bassinet. A bassinet for an infant is de facto a J-class lie flat bed -IMHO it is more comfortable for an infant than a seat. We have always travelled using bassinets for our under 2's. We flew from LON to SYD two years ago with our then 4 yr old and our DD2 who turned 2 on the day we arrived back in London - the dates of travel being determined by her birthday. We were travelling J class on MH but DD2 had the flatter bed. On one VS flight LON-MIA we were upgraded from Y to Upper Class (travelling on Con tickets) because the bulkhead seats were overbooked. We are travelling to Oz again next Feb in Y-class on BA, with 4 seats booked for our now 6 and 4 yr old DDs with our (then) 18mth old son in a bassinet. Because of the demand for bassinet seats the 4 seats are split by an aisle or in two rows, two in front of two. Not much use to you on this trip but just to let you know that BA also has a special child seat which may be more comfortable for older infants which can be used in place of the bassinet on the bulkhead (i.e. no seat required).

As you are flying QF here is what their website says about bassinet's:-
If you require a bassinet onboard, you must notify Qantas at the time of reservation. Bassinets may be requested for infants up to 18 months of age, but the recommended age limit for bassinet use on international flights is eight months. The weight limit is 11kg (25lbs). The bassinet size is 71cm x 31cm (28in x 12in). The depth is approximately 26cm (10.5in). Infants paying 10% of the adult fare who are too large for bassinets are not entitled to a seat and must be nursed.