Power meter = What you are doing
Heart rate monitor = How your body is responding to what you are doing
- You can push the exact same power in 2 different days and your body could respond very differently because you're flatter than usual, you are sleep deprived, you feel friggin' amazing, you are overcaffeinated, you got up from the wrong side of bed.
- Your heart rate could be elevated and you might think you are pushing yourself too hard when you aren't
Basically use both tools for what they are. HR can't provide you with the info PM can and PM can't provide you with the info what HR can.
If you absolutely have one tool, have the PM cause you can somewhat gauge how hard your heart is working. But have both tools. If you can afford a PM you can surely afford a cheap HR monitor.