I would check out reviews for both lenses at dpreview.com.
FWIW, I wouldn't use either. First off, if this guy is parting with the lenses, they aren't going to be anything to write home about. I know the Canon 24-70 is a lot more expensive, but it is so worth it!
If the Canon is totally out of the question, and you want to get a budget lens in that range with really good IQ, consider the tamron 28-75 xr di, which has a great reputation, and is considered by many to be comparable to the Canon L.