Everybody said that they have more or less the same IQ.
I checked photography-on-the-net and flickr. I feel 7D > 60D > T4i in terms of IQ.
Is it because the owners of 7D invest more in the lens so the outcome is better? Or the technique of them is higher level. Or I have something wrong in the judgement or I have 偏見?
Last week I picked up a Zeiss ZE 21mm f2.8. It's the best wide angle lens (I would call it a beast) I've seen so far, result is just amazing and pleasing...
The other concern is people are likely to have many EF-S (or other cropped) lens in their gear. It stops people (for amateur purpose) from moving into full frame.
In your example, people using crop bodies tend to pair with the Sigma 30mm f1.4 instead of the 35mm F1.4L. The result is just so close for output...
In my opinion, short to medium range zoom lens made by Canon is performing so bad (compared to the price tag). There are so many lemons for the L series. Surprisingly in my experience, the best zoom lens I can recommend in the Canon lineup is the 17-55mm F2.8IS. You won't even get a "level" upgrade even if you are using 5D2/16-35f2.8II or 24-70 f2.8 combo.
I think the sensors are pretty much the same. The different is the controls/focus speed of the camera. Unless, you speed is very important. I wouldn't invest money on corp sensors.