Since Apple eliminated the extra 9600M GT with 256MB in the 15-inch base: 2.53GHz? Do you think it's worth what Apple charges for the 15-inch: 2.66GHz model? Or is the 2.53 basically a glorified high end 13, with extra screen real estate? All things being equal, as a general rule of thumb, if you had to purchase a 15? What model would you choose?