I have researched these two and almost bought one but decided to move to FX and spend my $ on FX glass.
Short story - the IQ is the same if not better on the VC version.
What I found:
After reading endlessly and every article I could find the end result is that they have the same IQ and the VC for hand-holding gets you about 2 stops. The few tests that said the IQ was not as good on the VC version were using the lens on a tripod with VC ON which goes against the Manufactures recommendations. The Cannon version on one review site went bonkers with the test results and the review went back and re-tested with the VC off, mirror lock up on and the IQ scored higher.
What I learned from this experience: Reviewers need to write something to keep people coming to their site.
What is hard, is to review two lenses at the same time. When most are testing, they are provided a sample that they send back making testing both at the same time rare. Almost all the sites tested these lenses 2+ years apart, some on different cameras, Different software, and not always with the same settings.
Every reviewer site has preferences towards; Manufacture or 3rd party, VR(VC,OS) Vs. Non VR, Prime vs. zoom, FX vs. DX, etc. We even see the preferences on here between everyone. What I look for is when people do a comparison between lenses, if they are splitting hairs, that means either are a good choice.