You’ve been playing a while and you’ve been always using CoC and its worked out pretty well. Sure, you have to invoke the Golden Rule every now and then, but at the same time, you know the system, so why not use it for the fantasy campaign you’re trying to sell your players on.
Please don’t. I know many people think CoC is the greatest thing ever, but at the same time, that group of people haven’t usually played anything more sophisticated. They still like to think that the solutions of their youth are still valid solutions today. Would you still use the same computer you used back in the 80s? Probably not. These things have moved on. Why shouldn’t RPG systems? After all, they are of pretty comparable age.