you don't want to use the airplane's bassinet anyway, because every time you hit turbulence, you have to take your child out of the bassinet so they can stow it. so, basically every time Toby falls asleep, you have to wake her. we've traveled with our son and found the extra seat with the car seat to be the best option. and if you have a buggy that can hold the car seat, then it's even easier for the airport. (just check the buggy at the gate and then it'll be waiting for you on the other end as you get off the plane).
Good luck with your trip!

I realize this may be too late, but I'd like to offer a different
point of view. I think airplane bassinets are great -- and I wish
our boy were still light enough to fit in the Thai Air bassinets, which
are 10 kg max.

We did two long-haul flights (to US, Spain) with him, at 3 and 5
mos old. On both trips we ended up holding him in our laps for a lot
of the time, but it was still great to have the bassinet - to put him
down when he was deep asleep (and thus regain the use of our arms /
laps), to have some extra space to store/put things (toys, blankets,
etc.), and because if you get a wall bassinet then you are
automatically seated at the bulkhead. This means you get extra space,
which is nice.

We also did a trip to India when he was 7 mos, by which time he was
almost at the maximum weight and was over the maximum height.
However, since he doesn't stretched out to his full length, his height
was not an issue. For that reason, I wouldn't worry much about your child's
length/height. Even if his feet hang over the edge a bit, it's still
better than having him on your lap the entire time.

We haven't gone the car seat route, so I can't comment on that. For a
range of opinions, you might want to check out the "Traveling with
Children" forum at http://www.flyertalk.com

That said, with an older baby that moves / walks around, I expect a
bassinet would be less useful. Still, I don't think a baby that age
would happily stay put in a car seat either, so it's not an either/or
proposition.

If you do take the bassinet, one note of caution: make sure that you
are getting a wall bassinet. Some airlines have what they call "floor
bassinets", which are basically a cardboard box that they put at your
feet.

Good luck!

I meant to write before to say thanks for all the useful advice everyone
gave in response to this question.

Just to summarise for those who may be interested, the options are:

- book a bassinette and hope they have one and the baby fits and will
sleep (this is what I have decided on as he does sleep well at night
and I hope he will spend at least some time in there) Malaysian
Airlines has accepted the booking even though Toby is 10kg and 80cm long
so I think it does depend on the airline.
- hope there is a spare seat and the baby can stretch out a bit - many
people said this worked fine in practice as long as there were seats
available
- pay a child fare (which is expensive) and take the car seat, or buy a
special flying seatbelt which has been developed in the US, as they
don't accept the baby seatbelt as meeting safety standards there, the
website is http://www.kidsflysafe.com and they cost around $80 - the
advantage of this is you could take it anyway and use it if there is a
spare seat as an alternative to the bassinette

You can also take a light stroller all the way to the cabin door which
saves carrying the baby, and get it back when you land for use in
transit/at the airport. In Australia you have to check the stroller in
but they will lend you one for use in the airport before you board.

A few follow-up points re: the below --

- Different airlines have different height/weight limits for their
bassinets. The bassinet on Thai is rated up to 10 kg (I forget the
length - I think it's 67-68 cm or so, but we used it with no problem
when Daniel was at least 72 cm). As far as I know, Singapore Airlines
is the largest: their bassinets are rated up to 14 or even 16 kg (I've
seen different information in different places; it may also vary by
class of travel). Unfortunately, Singapore is also generally much
more expensive than other airlines...

- re: the CARES system (the seatbelt thing from the US), there are
some reviews here:
http://www.flyertalk.com/forum/showthread.php?t=637695. From what
people have said on that thread, it sounds to me like it might be
better for older toddlers (i.e., closer to 2 years old).

- Re: the light stroller, different airlines and airports have
different rules. If you're not allowed to gate-check the stroller in
Australia, you might see if they will still give it back to you at the
transit point (BKK, SIN, etc.), since we've found it really nice to
have a stroller at that point. This is especially true in BKK, since
the new airport can involve such long walks...