Call me mad... but cameras are a tool and the eye is all important.
I make the most photographically via my landscape and fine art prints, and I also cover events and photojournalism.
Up till recently I used a 1Ds III and a 24 mm TS-E II for the first and a 1DX with a 70-200 mm and a 400 mm f/2.8 I IS L for the second. Although quite happy with the TS-E II and the 1DX, I am feeling quite a bit let down by the 1Ds III. Files simply are not up to scratch compared to a 1DX.
Now a crazy thought entered my mind... how about going dark, and selling part of the gear to reinvest in a D800E (!!) and the fabled 14-24 mm, since I do most of my work on the landscape side?
The 400 mm f/2.8 has already been sold, at a profit. The fisheye, 135 f/2, 100 mm macro, 50 mm 1.2 might consider selling, keeping the 70-200 mm and the tilt shift.
Does playing both systems make sense? Canon have no higher resolution sensor, and the Nikkor ultrawide is a renowned lens. Now Canon might be considering a higher MP body at some point, but it is still total vaporware.
Whether you're mostly looking to landscapes or otherwise, I'm personally very disappointed with Nikon glass. Even with the D800E, it's too much resolution for a little sensor in the first place so it's noisy as all hell at anything above ISO 200, and the sub-par Nikon lenses rob much of the details. If you're looking for pixel-level quality for upscale-ability, wide dynamic range out of camera and a massive selection of the best glass in the history of photography, I would highly recommend looking into the Phase One/Mamiya medium format cameras. They've been working with 40MP sensors since 2005 or 2006; their newest backs range from 60-80MP. All of the sensors are upgradeable independent of the body and lenses, every Mamiya lens since the beginning of their 645 mount works perfectly with the digital system and the Phase One digital backs don't only "cancel out" the low-pass filter as in the D800E, they omit it altogether. Since I picked up the P45+ back and a handful of Schneider lenses to go with my old film/digital 645AFD body and Mamiya lenses, I've got the whole range covered from super-wide to telephoto. We tested my assistant's D800 (36mp) against the P45+ (39mp, flagship model from 2007) and the difference in the level of detail, smooth gradation and lack of noise is brutally in favour of the Phase system. Plus it does hour-long exposures with zero noise, as does their newest 60MP back.
Chances are you can find a used Mamiya 645 body on eBay for less than $1000, and if you can't find a used Mamiya/Leaf/Phase back at a decent price, most — if not all — medium format dealers (Vistek here in Canada, B&H/Adorama/etc. in the U.S.) allow leasing of new or used backs and lenses at pretty reasonable prices.
I think I should probably mention that I started my photography business with a Canon Rebel. I still come back to Canon Rumors almost every day, waiting for the day they announce something actually new — be it a higher-res FF or a AA-filter-less APS-C to actually get the most out of those tired, red 18mp sensors. But I ended up selling off all my digital Canon gear last year and now shoot Phase and Pentax almost exclusively (though one of my main 35mm film cameras is still a Canon). Here's hoping they've taken note of what Pentax, Fuji, Nikon and the others are doing and make a move to finally up their own game once again.
Best of luck, Charlesa.