Fibreglass is an older technology. The Fibreglass bows on the market that I'm aware of now are the very cheap Glasser bows, and that's about it. They were intended to fit a part of the market that was dominated by "disposable" wooden bows. By replacing them with Fibreglass bows, Glasser was doing their part for conservation, and they continue to have some really great programs, especially for school programs. They serve their place, but aren't really something recommended outside of beginning school programs.
Carbon fibre bows are a newer technology. They resonate a bit better, and because they have a different market, a lot more attention to detail is there. They are typically balanced really well, the camber is perfect because they can make it whatever they want it to be, and a lot of them are being fit with real Ebony frogs and silver windings etc. Generally speaking they have a much better reputation than Fibreglass, and they play pretty great. The biggest complaint about them currently is that they don't have the tone of wood bows, which is something very difficult to manufacture. If they can overcome that though, they will become unstoppable bows.
That's a fairly broad/generalized description of the two, but hopefully it helps.