If you are a fan of canon ergonomics i suggest you don't switch
I recently switched from nikon (d800) to canon (1dmkiv) due to better ergonomics for me and a lot more lenses available.
I really miss the resolution of the d800, but in time canon will come up with better sensors and resolution as well
. Lenses I am interested in (wildlife), canon offers more, with a bigger second hand market, and it seems they like the tc's better.
Some examples of what i did not like about the nikon system.
-Ergonomically , the camera felt like it was made for smaller hands than mine, and i really had to clamp it rather than hold it (even with the battery grip), The d7000, d7100, d600 and d800 all had this problem.
The d600 was better to hold than the d800 weird enough, never tried the d4
-Nikon position of the AF-switch ( to switch how many points you use , and afc-af single) , really awkward then you have anything bigger than a 24-70/70-200 on it ( even with the 70-200 it wasn't great).
- The position of the ISO button , basically same thing , it was on top (left side) of the camera.
When handholding a 200-400 on a d800, i found it akward to change iso or the af area, I could not set it to the buttons on the front (it was possible to change from dx to fx there). Maybe it's been updated or I was 2 dumb to figure it out, but ok.
-The glue of the filter holder actually melted away a bit , and the knob of the filter holder gets in the way when you bag the lens+body combination, it was getting lose after only 2 weeks of usage.
I have seen several nikon lenses where it had to be replaced.
-Less secure feeling with bigger lenses, due to ''smaller'' diameter lens mount, this is probably not even true, but very subjective view of myself
- Amount of pressure needed to press the af-on button on the back of the camera, again this may be different between cameras, but the ones i held (d7000/d7100/d800/d600), it seemed to me that more pressure was needed to press the af-on button, when compared to 1dmkiv/7d/5d etc.
This started to annoy me towards the end of the holliday, I was waiting for something to happen , for 2 hours I was constantly refocussing in anticipation of it to happen.
So in the end, even though the resolution was nice, the lens was great, and I am sure most nikon telephotos are close enough to their canon counterparts , the above things and the craving for more than 4fps made me decide to switch to canon.
The shoes fit better, even though I might run a little slower, I am confident I can keep on running ;P
And eventually canon will catch up on sensor performance, even if they are always 2 years behind I am still liking it better.
Also Nikon has it's own fair share of problems.
No 300 f4 vr yet, no 400 f5.6 , heavier 300/400/500/600 lenses.
They will be updated eventually, but if their 800's price is any indication, they won't be cheap.