I used Mi Bank 2 for 8 months and I also used Mi Band 3, truly said I did't saw any difference between these two expect Screen size. Mi Band 3 provide a battery life of 18 days, which is slightly lesser than Mi Band 2.
Mi Band 3 is minor upgrade of Mi Band 2 and you can't expect a lot of new features and improvements, for the price. There is a difference of more than 300 just because of Screen Size. If you want one I suggest both, take any of them both is good according to that price.
