I find the difference in shifting between the groupsets very noticeable when climbing. The tiagra which I've got on my winter bike is very clunky compared to 105 and that feels clunky compared to Ultegra. The newer 105 probably less so due to the trickle down from the better groupsets.
My alloy bike has been serving me well since I broke the carbon one. Its heavier, but it saw me through the Tour of Wessex and the Tour of Cambridgeshire. I would've been a bit quicker up the hills on the lighter bike, but the only thing that I missed was the smoothness of Ultegra shifting.