What are the advantages and disadvantages of these two aircraft configuration features? (I am referring to commercial-scale multi-engine jets only.)

1. Rear engines versus under-wing engines.

Rear mounted engines are closer to the fuselage centerline, reducing asymetric thrust in an engine out situation. However, you don't see really large engines mounted back there for perhaps a couple reasons:

1) Larger engines are heavier, so putting them at the rear of the fuselage could cause center of gravity issues. Wing mounted engines are closer to the cetner of gravity.

2) There could be some structural issues, maybe with the engine mounts.

If you look at the larger airliners, their engines are mostly mounted under the wings. This can cause some problems, like requiring longer (heavier) landing gear legs and a greater damage of foreign object injestion. Underwing engines are closer to the fuel tanks (both good and bad), so the fuel lines don't have to be so long or to run through the fuselage. However, the engines are also closer to the fuel tanks and that could be a bad thing in a crash because it might ignite the fuel. Below wing engines might be easier to maintain because they're closer to the ground.

Offhand, the biggest airliners that had rear mounted engines were the DC-10/MD-11 and the Lochkeed L-1011. They all had a single engine in the tail and two mounted under the wings. For planes with purely rear mounted engines, you had somewhat smaller planes like the DC-9/MD-8x series, the Boeing 727, Bac-111, and some others. The Russians did build a 4 engined airliner with all the engines mounted in the tail, but that was a pretty rare configuration.

2. Canard (elevators in front of wings) versus standard.

In theory, canard designs offer some aerodynamic advantages. For example, all the surfaces produce upward lift (the horizontal stabilizer on conventional configurations pushes down, so the wings have to overcome that force in addition to lifting the aircraft). Designed properly, the canard will stall before the main wings so the plane can't stall or spin.

In practice, the advantages are less pronounced and may be nonexistant for large airplanes. That's why you don't see any built in that configuration (Boeing proposed that configuration for their Sonic Cruiser but it was never built). One of the limitations is on high lift devices like wing flaps - it's harder to do that with a canard configuration. As a result, the wings might have to be larger to provide good low speed performance which would hurt high speed performance.

In the end, you can figure that the configurations that work best are the ones that actually get built. Look at all of the large airliners designed in the past 20 years and the ones being designed today. They all have underwing engines and conventional configurations. It's true that airlines tend to be pretty conservative, but companies like Boeing and Airbus have a vested interest in producing efficient planes. Boeing is investing billions to build their 787 airliner with the promise of it being about 20% more fuel efficient than current airliners. You guessed it - conventional configuration and below wing engines.