The fact it has independent cores for always listening to voice commands doesn't make the phone feel any faster. Also the GPU in the Nexus 5 is superior to the one in the Moto X.
Android has something similar, it's called Hangouts. On Android it'll handle text messages and send data-based messages to those with the app. https://play.google.com/store/apps/details?id=com.google.android.talk
There's an iOS version if you can convince your friends to use it. https://itunes.apple.com/app/hangouts/id643496868?mt=8
if you had checked the frequencies of the Asian version on the M8 you'd have seen it supports the same 700mhz LTE as AT&T/Cricket Wireless.
Honestly if the phone is a lot cheaper in HK i'd just buy it and use it on Cricket Wireless. As far as AT&T subsidiaries go they have some well priced plans for off-contract service. https://www.cricketwireless.com/cell-phone-plans
Well Sprint and Verizon will need their own version to begin with. Though if you want use it on T-Mobile you'll want to buy it from HTC directly or T-Mobile (the T-Mobile version will work on AT&T as well).
The Huawei Y300 has a much slower processor and GPU than the Lumia 520 and the Xperia M too. If you're going with Android you really want at least 1GB of ram (which the Y300 also doesn't have), anything less will be a horrible multitasking experience. You need to pay attention to this kind of important stuff because it's the difference between a good budget phone and buying a piece of crap.
Okay how about the Lumia 520 (Windows Phone can be pretty fun to use though not many games) or the Xperia M if you want a good budget Android. Both are cheaper than the Moto G and better options than cheap slow junky Chinese phones.
If you want a cheap phone, why not just get a Moto G, at least that will have a reliable warranty, be around the same price and will actually work with 3G networks outside of China