I want to buy a new MacBook Pro to replace my ageing 2009 model, but I can't decide between the top-spec 13-inch and 15-inch models. The screen size seems to be a personal choice, based on lifestyle, and I guess it's something I'm going to have to work out for myself. But display screen size apart, what is the difference between the two? As far as I can see, it seems to be between the different CPU and GPU. As far as the CPU is concerned, the 13-inch offers a "3.1GHz Dual-core Intel Core i7, Turbo Boost up to 3.4GHz" while the 15-inch offers a "2.8GHz Quad-core Intel Core i7, Turbo Boost up to 4.0GHz". Is this the crucial difference? If so, is it a big one? In what sort of applications would the 15-inch really outpace the 13-inch? Are there many situations where there would be no discernible difference? As far as the GPU is concerned, the 13-inch offers a "Intel Iris Graphics 6100" while the 15-inch offers a "Intel Iris Pro Graphics + AMD Radeon R9 M370X with 2GB GDDR5 memory" I don't know what the difference is here at all, or even what VRAM there is in the 13-inch. If I can get the "power" side of things straight, then I can concentrate on the choice between the two screen sizes.