It's really hard to put a specific number as to how much harder it is, it depends greatly on who you are comparing. One thing I can tell you is that the more muscle you have the easier it is for your body to burn fat since calories are burned through muscle maintenance. Since men tend to have greater amount of muscle mass, this is where the advantage is gained and getting lean to show abs is easier, however that isn't always the case since I know many women that are in better shape than the typical male.
Short answer: Yes women will find it much harder to develop a six pack.
Long answer: It really depends on multiple factors, but yes still.
First you should look at where your starting from.. if say your already really slim and want to just pack on the muscle, or if you need to slim down first, then get the muscle to show.
Also, biologically, women are 'designed' to store fat in their hip and stomachy areas due to reproductive qualities so its much harder to cut down fat in those areas.