I am new to the real estate photography as a photographer, however have been in real estate for quite a few years. Thank you for your informative website. I am in the middle of learning interiors photography and now trying to decide on equipment. My partner has a Canon 5DMkIII and has encouraged me to get one, however reading more about the 6D being lighter and a great camera, I am considering it although with your wide angle 10-22 or 10-18 review, I am facing the predicament of compatibility. My questions are: If I were to go with the 6D, what wide angle lens would you recommend? I read about Sigma but being a snob I prefer top of the line quality. If I chose lens first then camera, which camera would you recommend for me?
First of all, the Canon 10-22mm and 10-18mm lenses are not designed to work on full frame Canon DSLRs. For a summary of the lenses that will work on full frame DSLRs take a look at my lens recommendation page. The lenses that will work on Canon full frame DSLRs are down in the section titled "For Canon Full Frame DSLRs."
One of the issues with full-frame DSLRs is that the top quality lenses for full frame tend to be more expensive. One full frame compatible lens that is available for both Canon and Nikon is the Tokina AT-X Pro FX 16-28. It gets good reviews and is about half the cost of the top of the line Nikon and Canon full frame glass. You may want to consider the Tokina 16-28 FX if you are getting a full frame DSLR, it could ease the cost.
Don't feel that it is necessary to use a full frame DSLR to be successful at real estate photography. While you may be able to tell the difference between images shot with a full frame and those shot with a cropped sensor DSLR your clients probably won't be able to tell the difference. Sure, there are benefits that full frame DSLRs provide in low-light and dynamic range but these things are not going to turn all your images into great images. Notice that the March PFRE photographer of the month used a Canon T3i with a 10-22mm lens.
As far as which full frame DSLR to purchase if you are going to purchase one, both Canon and Nikon have quite a range of full-frame options. They all tend to be refinements of previous models. Go with whatever you can afford. And, don't forget used gear, the Nikon D700 and Canon 5DMkII are great DSLRs for real estate shooting even though they are both many years old.