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In terms of speed, ART is faster ( though consume slightly more storage ), but it's still in beta hence why hide inside developer option.
Dalvik could be as fast as ART, if you're using custom ROM that had implement dalvik library optimization, or flash yourself.
Battery seems similar but some people said ART had slightly better batt life.

i am using ART is ElementalX kernel

much smoother then dalvik, the only apps crash is titannium backup but after update to newer version it's working fine

ART is smooth and when I was on ART, most of my apps did not crash.

Now am on dalvik with the dalvik/bionic patches.

From my experience, battery more friendly on ART (for me).
With the patches, dalvik is as smooth as ART.

Because I want to use Xposed

I am back to dalvik. Just for the sake of going back. Haha. Later when google got ART stable then we might miss dalvik. Kekekeke

Cant wait to see what Google Now will be updated into. Now able to toggle Wifi setting etc, more coming ! 

Google Now voice control more

Wait for Google Search to be updated 

Launcher: Can try Nova/Apex/Action/Buzz/and many more.

Go flash then! 
No need wait. Just do a backup of current ROM and flash Miui then.
Don't like then restore backup. Settle 

damn... cakap 50 den 60 je..
once u drop the phone, den i'll "ehem" + "cough" at your phone lol

However, also depends on user usage. You may get +40%, maybe some user get less or not at all

Hi guys, anyone know that where I can get this phone around PJ area? Pefer walk in to phone shop rather than postage.

Basically Mi3 is N5 with mag-alu body and larger battery, and cheaper. However I've never like XiaoMi for one reason, MIUI OS, really hate it 
For that price, gonna sold out in 3 minutes maybe