i bought a 5D Mark3 and a 650D at amazon.
now i have 24 days left to decide if i keep the 5D Mark3 or the 650D
what puzzles me is that the image quality is, as far as i can tell, exactly the same.
i bought a epson R3000 printer and when i print the files i can hardly see any difference.
It's hard to express the joy I get out of a post like this
The reason you can hardly see a difference is because contrary to the exaggerated claims of some FF fans, there is hardly a difference at low to mid ISO. Properly processed and printed, a shot from Canon's 18 MP crop sensor in that ISO range will be indistinguishable from a shot from any of Canon's FF sensors. This is true at 13x19, 16x24, 20x30...any size actually.
At high ISO Canon's FF sensors start to show their edge. If you're shooting ISO 1600 or 3200 and making larger prints, you will observe improved IQ with the FF sensor. At 6400 and above the difference will be large. That said, Canon's 18 MP crop sensor makes very good prints through ISO 3200 with a little NR.
As to what you should do: I'm leaning towards the recommendation that you keep the 5D3 because of your trip. That's a once in a lifetime experience and the 5D3's AF, high ISO, and weather sealing will not let you down. I would maybe suggest returning it for a 7D if you need additional lenses for that trip and if you can use the money saved for that purpose. I don't know your budget so I can't make that call.
You mention wanting to buy a 24-70 zoom and a wider prime. What is your longest lens? If you are going to do any wildlife shooting on that trip, I would suggest budgeting for an additional telephoto. Canon's 300 f/4L IS and a 1.4x teleconverter (which could also be used with your 70-200), or a 100-400L.
If you can get the lenses you need and keep the 5D3, I would go that route. If you are going to be without a critical lens for the trip, consider the 7D plus the lens you need. Either way, try to keep the 650D for a backup. If you can't, take something as a backup, a P&S super zoom maybe. Again, it's a once in a lifetime event so make sure you're prepared.