F400 is Asian version
D855 is the international version
D850 is for USA AT&T carrier
D851 is for USA T-Mobile carrier
The difference between 16 and 32 gb model in terms of performance is minimal. Doesn't actually matter which one you get as there is a microSD slot.
the Asian version has an antenna for watching live tv, otherwise there should be no hardware difference between the models (other than the obvious 16gb/32gb). Inside the US each carrier has changed the software to their liking. For instance, the D850 (ATT) has Carrier IQ installed, the sprint model has the FM radio enabled (the others don't), the T-mobile version has an unlocked bootloader (the others dont)... etc.
If you buy a US version, you're going to have to pay to unlock it (few dollars) and I would choose the one that supports the radio frequencies for you carrier outside the US, as not all of our phones have the same frequencies.
Outside of the US from what I've read (which is by no means exhaustive) It sounds like the D855 is the one to choose, but other than the Asian antenna, the only differences are the packaging and what it includes. Some come with chargers, extra battery, headphones, etc.
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