The 15-inch comes with a discrete GPU (Radeon Pro 555), so if you're doing something like video editing or 3D rendering, then you'll definitely see better performance with the 15-inch. However the 13-inch isn't that bad, and as Rhinoevans said, it's mainly quad-core vs dual-core. For most everyday tasks, they will be very similar, but if you start doing tasks that require a lot of computing power, the 15-inch will produce better results.
Hey, what a coincidence! We both joined MacRumors on the same day.