Given your success rate with your current camera and focusing system/technique, I suspect that you would be disappointed by an upgrade to any other APS-C; you certainly won't get a "wow" inducing improvement in image quality, even if you prefer the focus system. As neuro and others have explained, you will see a significant improvement if you move to a 6D, especially in low light/high ISOs; but you should also see (albeit to a lesser extent) improved detail in good light, too - your EF lenses will perform better (go to thedigitalpicture and do some comparisons). You will also likely find that processing RAW images works better too - there's more detail to play with, which helps with repairing under/over-exposed bits, for instance.
(Of course, as someone else noted, there may be a wow factor you're looking for that has nothing to do with camera body performance and may instead be related to composition, lighting and so on but most of that's subjective. For instance, I think some of your photos would have benefited from being taken at a time of day when the sun wasn't overhead - but of course sometimes there's no choice - or otherwise being less bright, but that's just me.)
And while the 6D doesn't have as good a focus system as the 5DIII or 1Dx, it's not as bad as some keep saying - I use outer points in even low light pretty successfully when I don't want to focus/recompose (the one time I compared them directly, I found that if my 6D won't focus in low light on an outer point, chances are my 5DIII won't either), and while the 5DIII/1Dx have far more focus points to choose from, they don't occupy a significantly larger portion of the image area (for that you need mirrorless).
Anyway, as you seem willing to rent, why not rent a 6D and judge for yourself? You may (or may not) be pleasantly surprised.