Which do you think is more effective?
There are so many factors that can influence heart rate. Medications, stress, fatigue, not to mention that the formulas they use to figure MHR are not considered accurate. Research has shown that RPE/Talk test to be just as effective as monitors although, I do feel that monitoring has a place in training.
I like the talk test mainly because it makes your client pay attention to what is happening with them during their exercise session. HR is fine, but as the previous answers have suggested, the prediction are based on a statistical regression that is most likely not accurate for all individuals. The talk test is very specifically geared of your client right now.