Before buying any heart rate monitor you need to look at two main factors; the type of monitor, and the features each monitor possesses.
there are two types of heart rate monitors.
1) chest strap
2) wrist strap (watch)
Basically, a heart monitor sensor helps in determining resting heart rate and using heart rate tracking to help them improve or track their fitness.
When you are measuring your resting heart rate, a few BPM one way or another won't make much of a difference, but you want to determine if you are in a healthy heart rate range. Similarly, when tracking your fitness, your actual BPM is less important than getting into your target heart rate zone.