mhhenn,
To ask a couple of questions, why do you want to upgrade? Do you find something deficient with your current set up? Is there something you find desirable about the 70D?
There are a couple of online resources that evaluate cameras, TDP being my favorite one. But to answer your question, I went to DXOmark. Now, DXO is fairly well maligned as they ultimately try to compress a camera's image quality into a single number and also have a tendency to try to declare "winners" between camera bodies. Neither of these makes people happy. But, they do publish their test results, which you can find here for the three camera bodies in question.
I only did a quick scroll through, but at least in terms of sensor performance, the DXO test results for these three camera bodies make them look very similar to me. Their "overall" score is +/- 1 pt. If you look under "Measurements" you can see the more direct comparisons that I prefer. In it, signal to noise ratio,tonal range, and color sensitivity are very similar, almost having to look at specific ISO results to differentiate. I give dynamic range to the 550D and 70D. But, overall, these are very similar results. So I would expect very similar image quality from these three bodies. At least, it will take a lot of detailed analysis to tell the difference, which may explain what you are seeing in your evaluation.
So, if that is correct, it gets down to features. Is there a feature you want on the 70D in comparison to what you currently own? If yes, upgrade. If no, start buying flashes, tripods, and lenses or just go out and shoot with what you have.
Good luck,
Brant