[JJ_SO]

As has as I know, the D3300 has en extra internal WiFi unit, which you install once. And as far as I know, the D5300 has one internally included?

Correct. The WiFi unit for the D3300 (and D7100, P7800, D610 - there are two different units depending on the camera you want to use it with, another Niknonsense) is a very small piece of a Micro USB plug and a little box half the size of a regular USB plug. You can mount it like a GPS device - another inbuilt Canon feature, sigh.

Exposure simulation shows you on screen what the resulting exposure will be like according to the exposure settings. As far as I know, the Nikon cameras show an image exposed right for the scene, no matter what the exposure settings are. So you don't get to see what the result will be.
There are more oddities to the Nikon live view implementations, by the way. Not all Nikons have the same drawbacks in live view, the D3/4 series seems to be a bit better?

D800 does show you which exposure to be expected - when set up properly. Per default it appears like you described, but it can be changed to the behaviour your Canon already shows per default. I don't want to accuse the software programmers of Nikon, but they clearly force the user to read the manual. Twice.