Been using the Mi 3 for the last few days and each day gave me 7-8 hr screen time with 15-20% batt to spare. MIUI is well optimized for battery performance.
But in fairness, the camera is a bit of a letdown and I do get a bit more forced app closes. Maybe it is MIUI or maybe it is KitKat. HTC One (2013) still feels more stable even though batt life is about half and I get the hideous purple pix in dark colors which can only be eliminated by daylight or alot of flash. Seeing the Mi 4 doesn't interest me that much as 5.5 inches is too big for me and Xiaomi has to be careful with pricing. Plus it probably won't arrive in the Philippines until next year as the Mi 3 was just recently brought here last month. Companies tend to get greedy after success. Even local branded flagships here can get priced more than global brands. Early on it is cheap for companies establish themselves. Then prices go up. Mi 4 could be priced $100-$200 than the Mi 3.
I do think the Samsung Galaxy S5 still has the best camera I've ever seen on a smartphone along with the iPhone 5s especially for indoor photos with little light.