Nik, download a copy of
CCDCalc and fill in the values for the scopes and cameras you want to compare.
The field of view is the answer you are looking for.
I also have a 127ED.
A 5dMkIII would give you 129.8x86.5 arcmin. The ASI120MC would give 17.3x13 arcmin. That is about 2% of the area in an image from the 5D.
On the other hand, each pixel in the 5D covers 1.35 arcsec and in the ASI 0.81 arcsec so the ASI makes objects about 66% bigger.