Having 4 seats in Y is the safer option. Air travel itself is so safe that this may or may not be a criteria for you. There is little chance of anything going wrong but children are protected in car seats, those in laps aren't.

You may want to check out the thread on having children in premium cabins.

Do you need your car seats at your destination? Never check them as luggage so taking them on board is a better option. If you are flying off-season (and I believe, there is never a "low" season in Asia lol!) You might get empty places to put them.

Also look into whether either/both airlines will accept your seats. This is especially the case if the company you're flying is not the one where you bought your car seats.

Yet another aspect to explore is the actual seating in business. Some parents complain about "fishbone" and other seating arrangements that impede interaction with the other parent and child. Make sure that you can at least see each other and pass things between you during the flight.

Be sure to pack two diaper bags in either case, if you're separated.