I am on the same boat with you but on the business level.
In my college years, I also used my 13 inch MBP and it was pretty good to be honest. Bu when I wanted to work on two excel sheets or divide screen into two then the problems started for me.
I didn't have any external monitor for me to hook-up with my HDMI or any other thing, so I was bound to my screen all the time.
Now, I am checking out the new smaller footprint MBP,s and to be perfectly honest I am going to switch from 13 o 15. I don't know but feels lighter and smaller because of the footprint. I have made the comparison in Apple Store by dividing two screens in 13 and 15 side to side and loved the bigger screen which I can use with multiple things.
With every software becoming heavier with features and more every update you can easily see the difference of quad core or dual core. In my case, at Apple Store, I have witnessed when I fired up and divided screens for Excel and Word side to side and start typing formulas on Excel. On 15, it was smooth but on 13, I felt bit trapped and also crashed after couple minutes.
Extra GB upgrade definitely worthy because these machines are not like ur older machines that we can upgrade or play around anymore. Everything is soldered to logic board which makes them non-upgradable. So, if you can afford the extra bucks, go with the 16 GB; that's why I am doing too.
If it counts, count me in me +1 for 15 incher.
Hope it helps a bit.