Another consideration would be a used 1Ds original -- you'll give up some benefits, gain others and save a bit of money, especially if you are still "unsure"...
The 5D is an excellent camera, producing great files with a full-frame sensor, has great high ISO performance and a big rear LCD. And all this comes in a relatively compact and lightweight body. You can hardly go wrong with this choice.
By contrast, the 1Ds would be significantly larger and heavier, have about the same battery life but with a much larger battery and not nearly as good high ISO performance. However at lower ISO's (100 and 200) it delivers excellent files -- arguably perhaps even better than the 5D's -- is a 1-series body with the advantages of more focus-screen options and more robust and weather-resistant construction. It will also be about 2/3rds the cost of a new 5D which is why I mention it...
Both have excellent view-finders, deliver outstanding files (within the ISO constraints) and allow full manual control if you want it.
All FWIW only,