When I first entered the digital world, I knew I was possibly switching brands of camera -- my film camera brand was being acquired by another and the line was changing. I started out looking at, I think five different camera lines and quickly narrowed it down to two, Nikon and Canon. Not that there was anything wrong with the other three brands, I just liked the Nikon and Canon the best.
I then went in to my local camera store that has been around for almost 100 years (not a big box retailer) to look at them and feel them. I picked up each of them, had the sales person go over some of the features with me and tried taking a few pictures right in the store. I also tried making some of the exposure changes and other feature settings that I thought I might use. After doing this for each camera, I felt I had a clear winner. The pictures for both were excellent quality, both cameras had excellent response time. The only differences was in how you handled the settings -- exposure, bracketing, color temperature and other stuff. To me, one was more intuitive than the other -- it happened to work more like I thought. As I said before, there was no discernible difference in the quality of the two cameras or in what they could do.
I think it is important for you to go in to the store, ask to see a 5D and a D800, have the sales agent show them to you and try them. They will dig up a chip that will fit the camera to allow you to take a few pictures and look at them. I bet they would even put them on a USB drive so you could take them home and compare them (you would probably have to buy or bring your own USB drive). I did not do this last part (take the images home). I was not concerned with the quality of either camera. At that point it was the feel of the camera that was important.
There is no clear winner for all. Both take excellent pictures. Do I have a preference? Yes. But my preference is not about the quality of the camera but more that it works like I think. Friends of mine will swear by the other brand for exactly the same reasons. If I was aware of reasons why one was better than the other I would share them. I firmly believe this should be your choice in which one feels best to you.
Each time I add a digital camera to my collection, I look at the comparable models in the Canon and Nikon lines. So far, one brand has won every time -- it still is not about quality but merely personal preference. Go into the store and get the sales agent to let you play enough so you can pick one over the other. You will not save much if anything by buying one on-line. The prices should be same. If you find a place that ells one much cheaper than the others, I would be very afraid. For example, it might be an suspect import without warranty. Buying from Nikon or Canon direct through their website should be OK.
Please let us know what you do and how you like it.
. . . . T.