Your questions are roughly akin to deciding which type of car is best for you.
What is economical for one airline may not be for another; just as people with better credit can qualify for lower interest rates on their vehicle purchases. A brand new car (a/c) will have lower DOCs, but higher aquisition costs. The opposite is true for older equipment whether it's a car or an a/c.
For most of the 747's life, it has been purchased about half of the time for its range capability and not for its capacity, which has lead to the popularity of the 777.
WRT to the 767, seat configuration is one of thousands of options selected by the airlines, not Boeing. It's all a function of how an airline feels it can best utilize the available payload. And, like a lot of equipment, airliners are roughly priced according to take off weight, so an airline tries to buy what they need, and no more. On a 767 sized a/c, there is roughly a 50,000 lb variation throughout the product line, and up to about 25,000 lbs on a unique model like the 400er. The same with landing weights. I have seen many 727s recertified to land at higher weights simply by doing paperwork and sending a check to Boeing. Airlines buy the lightest a/c that will fit their needs. Are those needs near term? Or do you try to forecast demand in 10 years? One of the reasons NWA bought the A330 over the 777 was simply that the 777 was too much a/c in terms of a DC10 replacement. The 747-200s and 400s were both forecast to be around for a while at the time of that announcement.
As I've said before, the word "efficient" is the most overused term on this forum. It must be defined...How does the airline intend to use the a/c? At a min, one must state the mix of pax and payload over a specified stage length. Is ETOPs a factor for the route structure? If so, ETOPs cert costs big bucks on a twin, while it's not on issue on a 3-4 engine a/c. How about bely cargo and pallets? That's big. If it were simple, then the industry would only have 1 0r 2 airliner types with no sub variations...
BTW - "tris" and "seats" is plural, not possessive.