Thanks for clarifications.
Not too sure if 15 megapixels versus 18.1 megapixels really makes such a huge difference for A1 poster size. The two cameras are from different timeframes, but I suspect it should not be visible at low ISOs, all other things being equal. Naturally the 7D has an advantage for high ISO performance.
Some things to consider, given that your observations are wholly accurate -
1. How often would you print A1 poster size?
2. How often do you use ISO 3200?
The key issue here is, how good is this "good to have" capability of being able to print large and shoot at high ISO? If you feel that it is worth the price difference, and taking it along with the other differences between the 2 cameras (7D larger, heavier, supposedly better handling, better AF performance, additional features), then you can go for it.
I think most people are just trying to remind that many a time it is a user issue rather than a camera body issue. But it's also true that camera body can make a difference, across different generations, and within the same generation of technology. Cheers.