As you look at other reviewers of it that may not be knowledgable and merely comparing to what they got, the follow are the factors that influence calorie burn. I have no opinion about it, I just know they all attempt to do the same thing, many times using publicly available study research & formulas.
I'd confirm any reviewers are not merely commenting on the differences in calories which isn't helpful by itself, because tweaks to several settings can be done. Besides, different doesn't tell you which is better chance of accuracy.
Steps seen. Most devices in reviews seem to agree to steps by 3% or less, so usually not an issue. The ability to tweak settings to not see masses of false steps is sometimes a kicker depending on your daily activity. Like 100 bogus steps standing cooking, but the distance and calorie burn was barely anything - big whoop. 10K steps seen on daily gravel road commute with big distance/calorie burn - big deal. Steps seen with a stride-length setting leads to...
Distance seen. This matters for non-exercise daily activity time, when it is the most accurate method of estimating calorie burn. Most like Fitbit allow user correction when tested and needed to improve accuracy. Bogus steps may or may not lead to enough bogus distance/calorie burn to matter in the scheme of your week. Like if 5% or less, that is better than your food label accuracy and therefore food logging.
HR readability. Most have the same accuracy on people as they even use the same hardware guts. Meaning if your arm reads poorly with one device (either not showing high values, or cutting out in general), it'll read poorly with another (unless difference in 1 to 3 lights). This matters during exercise, when HR-based calculations are used. Sometimes this auto-starts if enough steps and HR goes above say 90, a common entry point into aerobic exercise.
Settings for HRmax. Many start with the sad but common 120-age for HRmax, and that value has a big influence in the HR-based calorie burn calc's. Fitbit allows adjusting it. Happens to adjust the HR-zones too, which that has no bearing on calorie burn. This setting and restingHR may be used to decide when to start using HR-based calorie burn.
Value for restingHR. This can be used in the HR-based calorie calc's, so how it's obtained or allowed to be set can matter, not as much as HRmax, but still. Traditionally that value was end of sleep right after waking. Useful too for tracking in order to see if lack of recovery, stress, getting sick, ect. Fitbit seems to include non-moving HR from throughout the day, and used in that calorie burn formula. I've had people note the exact same workout with same avgHR and no figures changed but restingHR, saw change in calorie burn. And there is research study formula for that which is public and many devices use.
Beyond that, it's how each shows it's exercise diary.
Many people do exercise for improving the body, not merely burning some extra calories.
Therefore to know you are improving requires comparison, showing progress.
That ability to do that easily and meaningfully is important.
This has a review of both in it by people that know how to compare. Each probably has written reviews too.
https://www.dcrainmaker.com/2020/12/massive-sportswatch-year-in-review.html
---------------------------------------------------------------------------------
Help the next searcher of answers, mark a reply as Solved if it was, or a thumbs up if it was a good idea too.
