Unless you are doing heavy graphics, a late 2013 15" MBP at 2.6 ghz limited to CPU side is 96% as fast as the current model. You can buy one with 16 gb ram and 512GB SSD for about $1700with a full year warranty from a famous reseller in Washington State. The graphics card of the current models is 2x faster according to Geek Bench, than my old integral Irispro 5200. But unless you are actually needing that kind of graphic power, its not a lot of use. People who differ will be telling me about stuff that shows the inadequacy of the Iris Pro 5200---which is a given.
The 15 has the 15 inch screen which is essential for my work. I've done the 12 inch screen and its just too small for the screen graphics work I do to see. Having a 8 thread i7 2.6(3.8 boost) is just gravy.
The MPB is simply not made for heavy gamers or for heavy raw graphics video users who need speed of action. But then doing something as simple as making a 55 minute movie with iMovie takes 6 minutes compared to 55 minutes on my 2011 i5 iMac.
Apple will never make a gaming machine or a desktop in a lap top. They are into sexy and thin. We all know that---at least those of us who have been with apple for a long time(30 years for me).
I was using a dual core x200 Thinkpad running Linux Ubuntu until January. I realized that while it did the work for me in speed, it was just not much fun. And so my new to me very very late 2013 15 MBP is 7 times faster, has more desk space, allows me to do my work with less steps, and bottom line, is way more fun to work on.
I laugh at the carrying around thing. I'm 68 years old. I carry three or four books, headphones, power brick, extra storage HHD's, sometimes a dvd burner in my back pack. The difference between a 3lb mac and a 4 lb mac is not even noticed.