I'm a D800E owner; I decided on the "E" because in early comparisons I saw a very slight (and please understand I'm using the word "slight" for a reason) advantage to images that contained complex fine detail over the stock 800. It wasn't a moderate or gigantic difference, it was slight, but to me, noticeable. Since both cameras can be provoked to produce moire, I personally didn't see spending the money by not getting the machine that performed the best, even if the slight improvement would mostly occur at apertures beneath F/11. A year later, this is my favorite camera ever in the 35 years I've been shooting in any format, from any brand. If as a young, poor 17 year old kid you had told me that in 2012 I'd be buying a camera that would easily match 6x7 chrome film and approach large format quality (which I have shot) in a 35mm package, I wouldn't have believed you, yet here we are.
However, after shooting with it for a year, using essentially near or at the very best lenses you can mount on it, I would have to say that if you are really desiring to see the maximum potential of what a D800E can do, you'll have to learn to shoot in less diffraction impacted apertures. In the old days I happily shot at F/11 and sometimes F/16, but these days I'm aperture bracketing, usually in the range from F/7.1 - F/10, often landing at F/8 or F/9, in order to still get close to what the cameras potential is. Search this sites article on "sensor outresolve lenses" and you'll see some information that shows you how much actual megapixel resolution is possible at each aperture due to diffraction - it's interesting, and may point you to a 20/24mp body if you insist on only shooting F/11 - F/16. I would highly suggest F/16 is one you stay away from on the D800E for those reasons.
Now, note in that previous paragraph I used the term "really desiring to see the maximum potential" and that's exactly what it means. I regularly shoot my D800E in the studio at F/11 and get wonderful results - I need the aperture for DOF reasons. But I know if I'm living at F/11 that I'm not getting the maximum potential out of the body and I'm good with that. However, for landscape work, I try everything I can to be lower, preferably in that 7.1 - 9 range if I can get the DOF for it. Think of it this way: If I gifted you tomorrow with a Bugatti Super Veyron automobile, obviously this would be likely the fastest car you own or have owned. And it would move. You could drive around a track with it and do very well. However, at the same time, if you then, after my gift, went and took a 12 week long driving course with a master race driver, you'd now go back home and be able to "see the maximum potential" of what the car could do, that you didn't earlier. This is a more coarse analogy than with the D800E, but the concept in my view is sound; you can shoot a D800E all day at F/11, but the smaller incremental gains you'd get in microcontrast and detail rendition from shooting a highly complex (in terms of detail) scene at F/7.1, F/8, or F/9 might be important to you. Or they might not be. So anyone who gets the camera gets a nice camera with a lot of dynamic range, but in order to really extract the sharpness/detail aspect of it to the maximum potential (there's that pesky potential word again) takes excellent craft, and excellent lenses.
In terms of the Sigma 35/1.4; I have one; it's replaced my Nikon 35/1.4G as I found it sharper, corner to corner, edge to edge, on my 800E in both close, moderate, and infinity distance tests. However, there seems to be a bit of sample variation as some folks took a few tries to get a good one. I was lucky - the sample I evaluated I then purchased and it's been stellar. That being said, I do feel it's performance is slightly (again, the word slightly) better in the closer moderate range, where it's bloody fantastic, as opposed to landscape distance/infinity, where it's just every day excellent. A lens that truly shines on the D800E is the Zeiss 21/2.8 by the way.
Now - at the end of the day, I generally don't advise brand shifts without solid, well thought through reasons. My "worry" is that if you are dead set on living in the F/11 and F/16 word that outside of the dynamic range improvement, you wouldn't really benefit from a D800E over a current 20/21mp Canon. And Canon will come out with a higher (perhaps even higher than 36mp) body with likely better dynamic range performance, so the question of how much you NEED (versus want) a higher rez/more dynamic range body has to be asked.
Good luck with your decision.