Obviously it is just personal, but I love Aluminum. I like feeling the road beneath me. As much as I may be tempted to get carbon for my next bike, I really think I will end-up getting aluminum.
Also - the CAAD 9 has a top tube that tapers as it reaches the head tube, while the CAAD 8 and CAAD 7 did not have that. This is probably the main reason for the stronger front. Plus, it is keeping up with the Joneses, especially as more and more Carbon frames have a sculpted vs. welded look.