p.2 #1 · Long and boring post: Which body would you upgrade to?
I sold my DIII and one of my 5DII's to help fund the cost of a 5DIII ...and I'm glad I did. I completely agree with Jeff above (so I won't repeat any of it).
I found my keep rate rose tremendously... focus accuracy is amazing. And fasssst! If you do action/sports and rely on AIServo, however, there's an issue with not being able to see the active focus points (see other discussions about that on this board).
If you do travel, as I do, the nice thing with the 5DIII is you can break off the grip to get a smaller, less conspicuous camera...great when you don't want to attract to much attention. I got my 5DIII at the beginning of May just as I was heading off to Europe and the grip wasn't available yet and I although I prefer a gripped body (for my large hands) I found the 5DIII was very comfortable to use without it.
Thank you for for those fantastic traveling pictures. My tip: Purchase whatever body you think you need, try to get devorced instead of beeing killed ( ;-) ) and TRAVEL!
p.2 #4 · Long and boring post: Which body would you upgrade to?
OP - I could've written most of your post, having many of the same "upgrade" considerations you're having. I currently own the 5d2 and 1d3. I recently just purchased a G1x and am finding it replaces much of what I wanted the 5d2 for. So the 5d2 is going up for sale. The only camera I'd ever dump the 1d3 for is a 1dIV, but the price on those has bumped back up recently so that move is off the table.
I tried Nikon (D700). I have nothing bad to say about the camera. I prefer the 1d3.
I think the G1x/1d3 combo works perfectly for me (more space in the bag for lenses!), but ymmv.