From the reviews I've read (this isn't first hand experience!) the ribble gets very very good reviews, and that seems like a very reasonable price for the frame and fork. I ride a full carbon road bike and it's so much more shock absorbing than full alu i've ridden in the past. I've never ridden an alu front/carbon back before though, so can't compare them 🙁
you'll almost undoubtedly notice a difference, esp. with different gemoetry, but whether that's a difference for the better or worse is one probably only you will decide! Do you feel the need for a more upright position? You could experiment with shorter, higher rise stems on the spesh to check that sort of position is what you are after. 15 quid on a stem is a much cheaper way of playing than 450 on a frame!!