I remember the wife and I put lot of money into upgrading our original "entry level" road bikes. Then I got my first Carbon bike (a Trek Madone 5) and I could not believe the difference! If you weigh anything, and have a large frame bike, frame stiffness is CRITICAL!
At the time I was riding regularly with a large group of nuts, and I could not believe how much more competitive that bike made me. I was no longer the caboose!!! The frame flex of the cheaper bike was costing me considerable time when climbing steep hills.
I thought the lighter bike would make a larger difference for my wife, because she is only 135 lbs (and 5'10" with same inseam as me). However, the stiffer frame did not make much difference to her. Everyone talks about the weight of a bike, but I learned (that for me), frame stiffness was far more important.