I think you are a bit biased towards the A340. There is no need to be. Because it is already a great aircraft. But some things i would like to point out.

1) SQ choice of aircraft is not engine confined. A few years back they were operating an almost all PW-powered fleet (B744 and A313). Yet they ordered the A343 and the B777 which are non PW powered. Other airlines might have different policies though.

2) A343 are efficient and cheap to operate. But they have their problems. They are underpowered compared to 744 and 777. SQ couldn't get them to take off quickly and assume cruising attitude fast enough. Sometimes a Sin-Paris 343 has to fly at FT 28 for a long stretch of distance because of its inability to power up quickly.

3) A343 cruises at 0.82M and the newer A345 and A346 cruises at 0.83. These speeds are slower than the 777 which cruises at 0.84M. This difference is considerable esp when compared between the 777 and A343. Air France uses a 777 for Sin-Paris which takes off later than SQ's 343 and yet arriving earlier (much to SQ's embarrasement of course).

4) Have you been into the cabins of the A340 and the B777 yourself? I have. The A340 has a 'normal' cabin. Cosy, quiet, and not too bad. But the 777 experience was much better. You may have your facts from Airbus.com. But the 777 ceiling is so much higher and much more elegant and stylish. Of course Airbus claims more headspace but thats only when the 777 overhead bins are open. How long are bins open on a typical flight? And things dun fall out easily as you feared. Try a 777 and you will see what I mean. In addition, there is more space per seat for a 3-3-3 layout in a 777 than the 2-4-2 in the A340. I took a window seat on the A340 and it was cramped close to the wall which curves in as if it was enveloping the passenger. The 777 walls are almost straight and there is more space between the wall and the wondow seat pax. The seats were also slightly wider.

5) As for ETOPS and routes which the 777 couldn't fly, that is true. But most of the long haul heavy traffic (e.g. cross Atlantic, cross Pacific, Europe- South east Asia) can be and already are operated with 777s. Actually, simple probability theory will tell you that the likelihood of one engine failing in two is lower than one failing in four. Of course probability aside, its the anxiety when the engine really fails.

6) Why different airlines buy a certain aircraft type is prompted by many undisclosed factors other than the performance we see on paper. One big factor is maintenance and the price of the aircraft. My father works in the maintenance line and he is sure that the 777 is much easier to maintain. But if the A340 is much cheaper in the first place, it may pip the 777 in a proposal.

Hope you would have a chance to travel on these two aircraft, if you haven't already. I like the 777 experience better, no doubt because of its spendid interior designing, even though it was more noisy. But ultimately, I love flying the 744 best. :))