RE: Pros and cons of the two main airliners designs
That argument seems to be over with the demise of MDC as both Boeing and Airbus use underwing designs. Also, the gear length is more related to the rotation angle required without a tail strike. Take a look at a 737, engines under wing and fairly short gear.
even the next level down appears to be mostly under-wing engines. Embraer has taken most of that market and the new Chinese design is also underwing. Part of it has to be aesthetics. The larger MD-80s and the MD-90 look more like lawn darts because the stretch needs to occur forward of the wing for balance.
John