I am currently riding a Giant Defy with rim brakes. I have no complaints about the bike and it fits me well. As my riding and fitness continues to improve, I am spending more and more time in the saddle, which leads to the inevitable thoughts of upgrades. I have read lots about how upgrading the wheels on a bike make a huge improvement and I know that wheels on my Defy are entry level, so that is one angle I am looking at. Nothing crazy, but maybe a $500 set of wheels and keep the stock wheels for winter/training use. Option B is to upgrade the entire bike. My LBS will give me a great deal on a 2016 Roubaix SL4 Sport which puts my into a similar spec'ed bike to my Defy but with a carbon frame versus aluminum. Does one scenario make more sense than the other?