I'm going to be buying my Mac soon, and I really want to know if there is a huge difference between [either] 13" model compared to the 15". From what I see usually the only thing the 15" does significantly better is games and graphics intensive stuff. I am not going to be using this for games except for the occasional play around in RCT3. So speed wise, applications, web browsing, and other simple stuff like that, is it relatively fast? Would I be able to have open multiple apps at a time without system lag on [either] the 13"? Thanks in advanced for the replies! 
[P.S - Screen size has no effect as I will be getting a monitor with it.]
[P.S - Screen size has no effect as I will be getting a monitor with it.]