miui8 redmi 2w

I have the difficulty in the choice between HM20140811(4.4) or WT88047(5.1) for my redmi 2W 1G upgrading to miui 8. I have flashed the last miui7 ver. Multi-6.5.26 (5.1). Therefre, I have two questions: 1. why in the last version of miui7 for redmi 2w, the android version is 5.1 for all version...