What is the functional difference between an e3D-Style nozzle, Makerbot MK8 and M6 Chinesium Nozzles? There is pretty much an ecosystem of two Nozzle designs out there that share the M6 thread on the coupler to the Heating block: 


the e3D "snub nose" or "shouldered" design.
the "Chinesium" nozzle that is often claimed to be some "MK8" or "MK10" without naming what item of which manufacturer is actually iterated there.


They seem to be derived from the Makerbot MK8 Hotend which uses M6 threading and not the MK10, as that uses M7 threading.



What differentiates the two and can one swap one for the other?
Differentiation

The main differences between the e3D-Nozzle family and the "simple" Nozzle are the wrench size, body length and thread length of the nozzle. In fact, I have come across 2 different "Chinese" styles of nozzle, a "big" and a "small" one. 

Comparison

For comparison, take a look at this photo, where I aligned the lower ends of the bodies to line up under the wrench needed to handle them.



In words, left to right:


Chinese Big M6 Nozzle


size 8 metric wrench, 4 mm thread length, 1 mm clearance, extends a minimum of 8 mm from the heater block

Chinese Small M6 Nozzle


size 7 metric wrench, 4 mm thread length, 1 mm clearance, extends a minimum of 8 mm from the heater block

e3D v6 normal (aka NOT volcano etc)


size 7 metric wrench, 6 mm thread length, 2 mm clearance, extends a minimum of 5 mm from the heater block



Replaceability

Chinese big to Chinese small nozzles in either hotend

Due to the dimensions, one can easily swap the big and small Chinese nozzles for one another. They are virtually interchangeable but differ in the wear patterns.

Chinese nozzles in e3D Hotend

The 3 mm shorter snout and deeper butting with the heatbreak of the e3D nozzle in its designed hotend make it hard to swap a Chinese nozzle into an e3D setup: neither does the thread allow to screw the nozzle in the right length sometimes, it also extends much further. To accommodate, the whole heater block has to be screwed about 3 mm more onto the heatbreak, then the nozzle gets screwed in. The result is equal in overall length.

e3D Nozzle in Chinese Hotend

However, the long thread of the e3D Nozzle allows it to be mounted in a hotend designed to hold a Chinese nozzle without trouble - the 3 mm of difference in the body are used for a longer thread and clearance between thread and body, resulting in the same overall length without changing the mounting position of the heater block on the heatbreak.

Internals

Stefan of CNC-Kitchen recently tortured a couple of nozzles for science and investigating wear and tear (video). He found out a couple of differences on the internals:


The Chinese nozzle had a non-straight pattern on the inside
The angle in the feeding cone is 60° in an e3D and 90° in the Chinese sample


Conclusion

The Chinese Style nozzles can be interchanged for one another. an e3D style nozzle with standard length (aka not-volcano) can be swapped in for any Chinese Style nozzle. A Chinese Style nozzle needs to have the heater block shifted if mounted into an e3D hotend.