The difference is that people will schedule themselves (walk or not walk, find food or not find food, etc.) around minutes and will be incredibly frustrated if the minutes are not counting down steadily and accurate to, well, to the minute. Displaying km allows one, knowing how reliable or not the particular route may be, to judge the situation for themselves and not put their faith in a sign that is virtually guaranteed to "lie" to them. I can't think of anything more annoying than a countdown clock that stops counting down when you could have walked or waited indoors or whatever else...it's like a download getting stuck at 99% for an eternity. At least displaying distance lets one know if the vehicle is moving or not. I'd say display both but it'd be confusing and more expensive.
The streetcar routes are often unpredictable, but almost every suburban bus route either runs at near comical frequencies or at lower but much more reliable frequencies (yes, it's true...some TTC routes run like clockwork). In some cases, having more info may be nice, but for the money and effort this will cost, I'd rather they put the slightest effort into maintaining proper headways or posting schedules at stops. The way I see it, posting next vehicle displays gives the TTC another excuse to provide not-so-optimal service, and they'll be able to get away with it because they'll be directly telling riders that they're waiting helplessly.