ELI5 Heart Rate Monitors - what types are most accurate?

Why do cardio machines require you to put two hands on them to measure your heart beat when a wrist watch can do it just with one wrist? What’s the difference between an ECG and EKG? Which types of monitor is most accurate?
... we explain like I'm five
Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: saltyson32, internetboyfriend666, blindedbythesight, iuse2bgood, vexx_nl, angryocelot and notwearingatie
To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter:
https://www.twitter.com/eli5ThePodcast/
or send us an e-mail:
ELI5ThePodcast@gmail.com