Switching and/or Adding?
As a shooter fully invested in a Canon system these days, I wouldn't find any incentive to switch to a Nikon system. Many many years ago I did the switch once when I was still in the more hobby than business position with a much smaller Canon system and found it hard to adapt after years of muscle memory and knowing the ins and outs of my Canon outfit. With a stable of hired clients and a large arsenal of Canon equipment now, I wouldn't even consider it.
When I mention system, I am talking about more than just a camera and a few lenses. I'm talking about backup bodies and backup lenses, a truckload of lp-e6+lp-e4 batteries, a boat load of 580exII+600ex-rts , lr workflow, express digital remote capture + icc profiles for location dye-sub printing, remote lighting/trigger setups with pocketwizard flexes, cp-e4 remote battery packs, n3 trigger cables for multimax remote capture with off camera flash+shutter release, wft-e4/wft-e2 grips, saved CSD profiles for my mk3/mk4 each configured with a ton of custom settings, drivers and software configured on my desktop and each of my location notebooks. The list really goes on and on. Some of these items may seem small, but each of these things solve a specific use case for a specific type of job, and would have to be addressed in its entirety for a switch to happen. For me, the decision was easy; I upgraded my mk2s to mk3, pre-ordered a 1dx and upgraded all of my flashes to take advantage of the new body specific 2012 features.
Most of the Nikon/Canon comparisons here show relatively modest differences between these newest bodies. While I agree Nikon's Sony sensor is clearly ahead of Canon this time around, but is it really enough to switch or add?
In fact, these days I wouldn't even find much incentive of adding a Nikon system to my Canon kit either. It just doesn't provide me enough of a differentiating factor to make a difference. I'd have to acquire at least 1 zoom and 2 primes with that combination and that would easily run into the $10k range. There are so many other alternatives that can actually improve my work aesthetically. Something in the way of adding a 300 or 400 2.8L, or reaching into the pockets a bit deeper and adding a Hassy digital medium format system would give me a significant tangible difference. Hence is why a H3D (no H4D) is still on the roadmap as an addition to my kit, but Nikon isn't.
I can't help but be a bit curious as to the thought process of those that are considering switching or adding a D800 to their existing Canon kit?