I do both.
I start an exercise with heavy reps, and stop it when I can only complete a few forced reps.
After completing those forced reps, I pick up a light weight,
and crank out as many reps as I can until my arm is swollen from the pump.
I call this part of the set a "burn session."
If you are only doing heavy reps, you aren't breaking down the muscle as much as possible;
you are still left with enough energy to continue on a lighter weight.
Also, this method makes it harder for your body to adapt to one type of training.
To further compound things, I do explosive reps on alternating weeks. I call them explosive reps,
because I aim to jerk the weight as fast as possible. I do this for speed, and as mentioned, to mix things up.