Battery life. This is one of the biggest selling points to me. I'm replacing a Fitbit Charge 2 that would last around a week or less. Previous to that I had a Mi Band 1S and one of the best things about it was how long it could go between charges. I'm really looking forward to getting my Mi Band 4.
I ordered a Mi Band 4 not long ago and it should get here next week. I'm looking forward to it.