What's better for a gravy - flour or cornstarch?

I'm making a beef pot roast in my slow cooker as we speak. My sauce is basic: a can of French onion soup, a can of beef broth, a can of beef consomme, a little red wine, and various seasonings. I went with cornstarch last time, and while it tasted good, I had trouble with clumping. Would flour be less susceptible to that? What about Bisquick?