Originally posted by RRAM
the Do-335 had two engines, one pusher, other puller. The after propeller was known to have a quite low efficiency.
The Do 335 actually was, according to British test pilot Eric Brown, "measurably faster" on the rear engine alone than on the front engine alone. Dornier planned developments of the Do 335 with the rear engine only, in some cases with the addition of jet engines.
The pusher configuration turned out to be the more efficient one, despite the disadvantage of disturbed flow entering the propeller disc. Hence the willingness of the USAAC to experiment with it in the R-40C competition (which produced the XP-54, XP-55, and XP-56), all pushers, and the later XB-42 bomber. Hence also the German preference for pushers in what was intended to be their last generation of propeller fighters. However, technically pushers tend to be hard, requiring very stiff aft fuselages, long extension shafts, or rigid tail booms, and problems such as ensuring sufficient cooling on the ground and a safe way for the pilot to leave the cockpit in an emergency are serious. The R-40C program was plagued by engine troubles and the Germans did not have any more luck. Often the additional weight of the configuration defeats the aerodynamic advantages.
The canard configuration was also the more efficient one. On a conventional stable aircraft, with the c.g. in front of the center of lift, the tailplane gives a download, which has to be counteracted by additional lift from the wing, all at the cost of extra drag. (Which is why unstable or marginally designs are popular now, even for airliners.) The canard surface has the advantage that it can balance the aircraft whilst generating lift. The potential disadvantages are instability -- if the angle of incidence increases, the AoA of the canard also increases, so to the nose raises even more -- and a tendency to stall much easier than a tailplane, because the canard has a positive angle of incidence. The stall characteristics of the XP-55 were quite bad.
Manoeuverability probably doesn't enter much into it, with WWII aerodynamics. Modern canarded fighters use vortices from the canards to 'energize' the flow over the wings, but this was beyond the aerodynamics of 1945. Having a better lift/drag ratio would have been useful, though.
