Pros: Guaranteed monthly income and reprint royalties. All monies on a creator-owned Image series are earned on the back end, and there is a chance that you will make nothing from the series.
Cons: Loss of rights to other media. Warner Bros has right of refusal to develop your property in other media, merchandising, etc. All Image creators working on creator-owned books hold the rights exclusively to their property.
The easiest way to boil it down is that Vertigo is a partnership between the creators and DC with shared rights, but Image is truly creator-owned with all rights held by the creator(s). The risk is much greater in the Image model, but if the book is successful, the reward is also much greater. That's why most Image books are by new creators trying to break into the industry or established creators who have a dedicated fan base.