rent maybe first…I think if you're using a film camera now…that's the best image..(digital is for speed…work flow..work)
For a digital image you can add film grain in post processing, so that you will have comparable results. How then would film give you the best image??? Just try shooting film at ISO 800 or why not 1600 or 3200. You can get the same or better result using a fairly old digital camera. At ISO 50 or ISO 100 you can get results comparable to a digital camera, but that's under ideal conditions.
Do you develop your own film to make the most out of it? Post-processing a digital image is available for everyone on their home computers. You even get several tries to make it good. It's not only the speed, which has gone up. Image quality has gone places, too.
Back to the original question here's my advice in simplified form:
In case you need the silent shutter feature of the 5D Mark III, then this should be what you purchase.
If you need speed and/or optimized useability then you should go for the 1D-X.
I use both. 5D Mark III can serve as an excellent backup body for 1D-X.