bdunbar79 has some good experience and insight to the question. Both would be good for what you want other than the tungsten issue he mentions, so it comes down to cost and features.
What are the prices of a 1dm3 Vs 5dm3?
Do you need bullet proof build?
As far as portraits and landscapes the edge may go to the 1dm3
Do you need new features of the 5dm3?
Is weight an issue?
Given what you want it for high iso isn't needed.
Both have good focus but the edge would go to 5dm3 and its spot focus as well as live view focus for portraits that allow you to see extremely zoomed in on the 5dm3.
You just need to do a checklist of wants and see where things end up.
Yeah you're absolutely right. For me, it came down to selling the 1Ds3 or 5D3. (I had 2 x 5D3 and 1Ds3). I automatically selected the 1Ds3 as first to sell. I could have sold both 5D3's and kept 1DX/1Ds3 but I instead chose to go 1DX/5D3 combo. The 5D3 overall is just better. I'm sure you know this as you shoot quite a bit with the 5D3 too. The sale was just an automatic and obvious choice in my head right away for some reason. 1Ds3 images can get ugly above ISO 800 and 1600 and let's be honest, to most people the 5D3 and 1Ds3 images look about the same from ISO 50-400. Only us pixel-peepers can see the difference in color rendition. Landscape shots do look a bit better if examining very closely. But yes, overall the 5D3 wins.
Even though I love the 1DsIII the 5DIII has to be the better all rounder camera - in my experience th 5D2 was better all round, but the IQ of the 1DsIII at 50-400 is something else. As I have other bodies I can afford to keep the 1DsIII for those moments - especially now it has a new shutter and is pretty much mint.
My 1DX comes next week so it will be interesting to see if it meets up with the Canon claims of 1D4/1DsIII merging.