The lord can still be instagibbed by S:10 weapons (denying him his WBB roll).
That being said, you're only sort of correct.
It may be the case in most situations encountered during the game that a better armour, lower toughness unit is more survivable than a worse armour high toughness unit, but don't let that mislead you.
In actuality, each part figures into the equation with exactly the same weight.
Take the instance of guard and orcs being hit by lasgun fire.
Guard take 1/2 of the hits in wounds and 2/3 of the wounds in kills, or 1/3 of the hits in kills.
The guard have a better save, but they die more to lasgun fire.
What you'll actually find is that the model type whose positive aspect (that is, its stat that is better than the one you're comparing it to) produces target numbers farthest from the average roll will perform the best (presume the variation between total relavent stats is zero, and none of the game-imposed limits are reached--such as the instance you mentioned where both get wounded on 2+ despite the difference in toughness)
To illustrate, consider the sisters and the scouts. The scouts were better than the sisters in toughness, and a 4+ (1/2 chance) was needed to wound them. The sisters were better than the scouts in armour, and there was only a 2/3 chance of a wound getting through. The 2/3 was farther from the mean than the 1/2, and the sisters perform better.
You'll see the same thing is true in the case of the guard.