Since a 50-megapixel 5DS/R won’t ship for ~3 months, it does not exist in market terms. On that basis, it’s laughable to see it proclaimed as the new champ. Nikon is surely not idle, nor Sony. Let’s see what they come up with.
The Nikon D800/D800E was excellent two years ago (still is), but the Nikon D810 now 'rocks' as the king of the hill, being superbly balanced in its resolution, dynamic range, Live View, EFC shutter, and overall superb file quality.
Those who went with Nikon when the D800 debuted will have enjoyed the dynamic range and the megapixels more than three years—more since it will be at least June before Canon ships its new 50MP 5DS/R models with the same dynamic range as the existing 5D Mark III, and as yet unclear noise properties. The dynamic range does make me scratch my head in bewilderment a bit; it’s not competitive. I love being able to shoot the D810 in just about any lighting, confident that it can handle the range. Often it has a stop or two to spare, to my ETTR consternation (poor histogram computation relative to raw data on sensor).
I expect Nikon and Sony to offer ~50MP cameras this year, and I expect a superior sensor to the Canon offering, my thinking being that Sony sensors are the best available today. It just doesn’t compute that a 50MP evolution from the current 36MP sensor will regress in capabilities. So even if a Sony 50MP sensor improves in no way but resolution, it would be the clear-cut winner.
So the equation three years later here in mid 2015 looks scarcely different than 2012.
Still, the Canon lens line is very appealing for some types of work (tilt/shift* and also the new 11-24mm zoom and certain long telephotos). For specialty work, the camera should be thought of an accessory (barring real specialty stuff like medium format and view camera movements and special sensors, etc).
* AKA tilt/shit when shooting on a steep slope and the tilt involves the tripod a tad too much for gravity to bear. Which do shit in the woods.