I would love to own one of the two, personally, $6,799 for the 1d-x is a great price considering what it can do, and that it is full frame, but theres nothing wrong with the 1d Mark IV. Who is HONESTLY going to use 12-14 FPS aside from Olympic photographers and professional studio users who work constantly??? Even though it will be a MAJOR improvment to the 10 FPS of the 1d Mark III and the 1d Mark IV, there isn't a need for the camera to even do that much for a regular shooter who does portraits, weddings, and personal photography. I thought it was going to be a have to buy, but when I seriously sat down and thought about it, there isn't a need for me to purchase the camera for any means.
The 5d Mark III is another story though. This camera is very awesome. But its upgrades from the Mark II, for me, don't justify he $1,500 price upgrade. If one was doing a ton of weddings, then this camera would be the GO TO camera along with a 70-200 f/2.8 IS and a 24-70 on a 5d Mark II or a 7d IMHO. THe 5d3 is probably going to be a camera I save up for, I currently only have the 7d gripped with a couple L lenses, however, I'm stongly considering purchasing the 5d3, pairing it with my 70-200 and the 7d with the 24-70, or vise versa, depending on how I feel with each lens on each camera. I just have to convince myself that the 5d Mark II isnt the camera I need, and I will probably wait till they perish the 5d Mark II before I get the 5d3. (I dont like using gear they dont make anymore, ignorant, but true).
For aquarium photography, I think the 60D is the best camera because its got so many "user friendly" features like the articulating screen, its basiclly a suped up rebel that offers a lot for the price.