from April 2011, tagged GeekBench
The MacBook Pro 15-inch has a 16% larger screen and is 24% heavier than its smaller MacBook Pro 13-inch sibling. This extra weight makes it difficult to carry around by its palm rests while open, which is significant as a majority of users don't leave their computers on the desk anymore. While the 15-inch is twice as fast in Geekbench scores, most users will not notice the difference unless performing CPU intensive tasks.
The 13-inch is the perfect size for most users. It has just the right amount of screen real-estate while maintaining its true portable nature. Mac OS 10.7 full screen mode, which will be coming out this summer, will help maximize its screen. Apple offers three different 13-inch MacBooks for different customers: MacBook, MacBook Pro, and MacBook Air.