Personally I tell my students that the priority in choosing a field is well behind not letting the engine fail in the first place, and if it does getting it going again and if you can't get it going, maintaining control of the aircraft at its optimal glide speed.
A 60 knots to 0 knots uniform deceleration takes about 25 feet. Now obviously it would be hard to do that in the real world, but the take away is the secret to a survivable forced landing is to have the airplane have even a small ground run. The killer accidents are the instantaneous stops from flying speed that occurs when the airplane hits an immovable object or the ground in a steep nose down attitude. Since the condition of the airplane when it stops is immaterial, the only consideration is to avoid injuring the occupants, what the field looks like doesn't really matter, what matters is having the airplane touch down wings level, nose up, at a slow but in control airspeed, and most importantly at the spot of the pilots choosing.
You obviously have to tell the student something about field selection during the PGI, so I tell them the criteria are, Close, Open ( ie fewest obstructions on the final approach) and Clear (of any big obstructions that could cause the killer sudden stop).
I emphasize close is always better then farther away. It is much better to comfortably make the close crappy field than arrive just short of the perfect field that is too far away. I also emphasize the importance of having the aircraft actually touch down at the chosen landable area. Most students are high on the approach and there is a real risk that they will float over that reasonable level, flat patch and then smash into the tree/wall/building at the far end of the field with catastrophic results. Therefore if it looks like you are going to be long then make the aircraft touchdown by retracting the flaps and/or smashing the aircraft onto the ground.
Ps: I promise I will try to turn over a new leaf and be less confrontational on avcanada, for this year at least



