I'm wanting to upgrade to a newer rMBP and have a few questions. I'm debating on getting either a 13" w/ 2.9 dual core or 15" w/2.2 quad core. How much of a difference is there between the 13" 2.9GHz dual-core Intel Core i5 and the 15" 2.2GHz quad-core i7? In layman's terms, what is the difference between a dual-core and quad core (besides being 2 and 4)? Wasn't one of these supposed to be a Broadwell? I am fine with a 13" screen size, but I'm mostly concerned with the processor. I am going to be using this for recording music via Garageband, some light video editing, and some other things. Thanks for your assistance!