I've been thinking to move to Full Frame from a while now, and I still don't know which camera I should get, a not-that-amazing-but-cheap 6D or a top-notch-camera-but-expensive 5D Mk III. If the budget wasn't a problem, I'll go straight to the 5D, 100% viewfinder coverage, 1/200 of sync speed, plenty of cross-type AF Points, full magnesum alloy body, and so on. But the budget is a problem. Especially because apart the camera, I'll need to buy a good wide lens as well, as my beloved Sigma 10-20mm I use now will not work on a full-frame body, and as a landscape photographer I cannot live without a wide lens.
My wallet says "take the 6D", my hearth says "take the 5D Mk III", my bank says "What about changing hobby? Like doing nothing and give us more money?" my girl says "I don't really care, but what about you buy something to me instead? Like clothes?".
We will see, 6D, 5D Mk III, clothes, it's hard to say what I will buy in the next month or two. In the meanwhile if you have the same problem, you can watch this video from Matt Granger of PhotoNewsReviews which compares the two cameras, hope it helps!