Nexus 5 models 820 and 821: What's the difference!!

only thing to consider when making your purchase is mobile radio bands

Just like in many cases, the Nexus 5 isn't one but rather two different phones — the D820 and D821. The former is made for use in North America, the latter in the rest of the world. While this isn't nearly as confusing as some phones, there are still a couple of differences between the two models. Externally both models are the exact same — the case, buttons, camera and design are indistinguishable. You have the same options of storage and colors as well, which is great.
On the inside, things are almost identical. You'll find the same spec sheet for the screen, processor, RAM, sensors, Wifi and everything else. All indications are that both the D820 and D821 run the exact same software build as well — Google seems to have gone all-out trying bring "one phone" to the entire world. The only tangible difference between the two models is mobile network bands — this makes sense, as it's basically impossible to fit every necessary radio into just a single model:
North America (D820) model:

• GSM: 850/900/1800/1900 MHz
• CDMA: Band Class: 0/1/10
• WCDMA: Bands: 1/2/4/5/6/8/19
• LTE: Bands: 1/2/4/5/17/19/25/26/41

Rest of World (D821) model:

• GSM: 850/900/1800/1900 MHz
• WCDMA: Bands: 1/2/4/5/6/8
• LTE: Bands: 1/3/5/7/8/20