Does the N3 socket/connector provide any advantage over the E3 counterpart or is it just a way to differentiate the more expensive cameras from the other ones and possibly make customers pay for the remote triggers twice?
There is no essential difference between the two types of connections other than the shape and arrangement of the connectors. The N3 socket on the camera body is probably easier to make weather sealed than the E3 socket that is basically a 2.5mm stereo mini jack. Both have three wires: a ground, a half press, and a full press.
That is, in fact, the case with almost all DSLRs that have a connector for a wired remote. For more about how theses connections work, see this answer and comments.
Amazon once sold such an adapter in the U.S., but it is no longer available. However, many of the third party accessories that are available with one connector are also available with the other and are usually fairly cheap. There's even a TC80N3 knockoff that comes with adapters for almost every imaginable camera that uses the basic three wire shutter release.
I haven't used them at all, but to me the N3 seems more sturdy. I know the 2.5 mm connector is a bit weak, but not how it compares to the N3.– HugoJan 30, 2014 at 14:21
I've never broken a 2.5mm E3 plug, but I have broken an N3 plug on the end of a cheap generic cable release. On the other hand, every Canon body I've owned except one uses the N3 connector, so I haven't had much opportunity to destroy an E3 (2.5mm stereo mini plug) connector. Jan 30, 2014 at 14:29
I see. I take it as the possible (non)sturdiness isn't an issue then– HugoJan 30, 2014 at 14:49
1I think both types of connectors can be either cheaply made or made well. The N3 socket on the body allows it to be tighter in terms of weather sealing than a 2.5mm female stereo mini jack, which is essentially what the E3 socket is. Jan 30, 2014 at 15:09