There are some aluminum designs with carbon stays (either both seatstays and chainstays or usually just seatstays) which will give you some vibration and small shock damping in the rear. You might wish to test ride them as well. Personally, for comfort and ride quality in general, I prefer steel, titanium or carbon fibre but one has to keep in mind that material is often times less important than the design of the frame, the manufacturing processes (especially with CF) involved and the quality of the wheels.