I made an experiment using Xiaomi Mi Band 1s and Mi Band 2 at the same hand simultaneously for a couple of days.
The general results are presented below. There You can also find the links to more specified conclusions as well as to the raw data 
I hope You will enjoy the reading and please feel free to ask questions as well as to add Your own comments and observations regarding two bands. I believe that will be helpful for the future Mi Band… users.
My conclusions.
During the whole experiment Mi Band 1s always showed more steps than Mi Band 2. The duration of sleep measured by each band was quite similar. The main difference was connected to the deep and light sleep. Mi Band 2 has the tendency to show longer deep sleep. I made an experiment for six days and Mi Band 2 showed longer deep sleep four times.
The conclusion is quite simple: Mi Band 1s makes people walk a lot while Mi Band 2 users begin to sleep deeper
I compared heart rate data measured continuously by each band during activity and sitting down. In order to do that I used MiHeartRate app that can be downloaded there:
I did not check one-time measurement via Mi Fit app or with the help of the button on Mi Band 2.
Generally it can be told that heart rate data measured by each band continuously is quite similar. For people willing to use the band during workouts I have written the separate article on measuring high hr by MiBand 2.
I did not compare notifications. Thus, I would encourage people interested in them to use another source of information on it.
Of course, an obvious advantage of MiBand 2 is that it has a small monitor. You can check time and after the latest release the data, the amount of steps You made etc. on this monitor.
Please let me know about Your opinion on Xiaomi MiBand 1s and MiBand 2 in the comments below.
Как умный будильник на основании 
Приложение Mi Heart Rate – be fit работает с Mi Band 1s и 
Congratulations! You decided to take more care of Your health, to try something new, to become more conscious of Your body, to spend more time with Yourself and maybe with other people in new sporty circumstances, to do something pleasant and healthy.
You like going in for sports, You already experienced the pleasure of quick evening jogging and enjoyed feeling Yourself strong and healthy after a series of workouts. You have already chosen healthy life-style and everyday physical activity is important part of Your life. What can be done more to be healthy and sporty?
You just should download and switch on MiHR app, tap the start button and You may be concentrated on Your training, on Your breath, on surrounding nature or city landscape. MiHR app tracks Your heart rate and informs You whether You Your heart rate is too low or too high.
Going in for sports and keeping healthy diet can strengthen Your health and regulate Your weight. How intensively to train to be sure it is the most efficient for Your appearance?
reaching Your goal or raise/ reduce Your workout intensity if necessary;
Your heart rate can show not only the level of Your physical activity, but also the extent of Your mental and emotional burden. While performing diverse duties we experience stress. It happens that the requirements are so high and the effort is so intensive that stress is persistent. Sometimes one may not pay attention to his or her own needs and not recognize the signals given by his or her own body. That may be dangerous and cause serious health problems.
