Other than low light performance (which you mentioned) and bokeh, there's a significant difference in the overall perceived resolution between full frame and cropped sensor cameras. For example, even the old 12.8MP Canon 5D classic would in most cases out-resolve 18MP Canon 7D, given the same lenses.
Now, back to your question:
The perceived resolution (per DXOMark) of 7D (same as 60D) with Sigma 30 f/1.4 is about 8MP, not reached until f/5.6. It's actually much worse below f/2.8
The perceived resolution of Canon 5D mk II (close to 6D) with a cheap 40/2.8 is 15MP and it's available from f/2.8
How much IQ advantage will this have over 60D + Sigma 30mm 1.4 ? With and without speedlight. Price difference is significant.
Last edited by alex356; 02-24-2013 at 07:18 PM..