IMO, stiffness and weight are overrated if you're just talking club riding. Vibration damping and ride quality should be a higher priority. A more comfortable, damped frame allows you to keep pounding out the miles while an ultra-stiff racer may become tiring. Frame materials are only part of the story here, carbon frames can be smooth and comfortable (Roubaix), steel frames can be stiff and lightweight (English).

My personal experience is that I vastly prefer the ride quality of my cheap-o steel framed bike to my super lightweight carbon racer, especially on the longer rides. The carbon frame is extremely light and as stiff as a bridge truss but it really beats me up on the rough roads around here. The steel frame glides along, smooth and silent. Also, it's probably not as stiff as the racing frame, but the steelie is hardly a noodle when I'm out of the saddle. Modern steel alloys are pretty good. Again, frame material is only part of the story here...

I can't fault the carbon bike, it's meant to be a no-compromise race bike. But, there's a reason why most people don't drive track cars on the road...

One note: I just bought a titanium racing geometry frame. Never ridden one before, we'll see how I like it.