For web development / light photoshop, illustrator work the 13 (6 months old) has been working fine for me. I came from a 15 and I never left the house without my bag. With the 13 assuming a full charge, I just grab it and go. I gave my old (3 years) 15 to my wife and though she never takes it anywhere without a bag either, she's never complained about it's weight.

Not sure if the 13 can push a 4k monitor but I've had no problem connecting 2 additional non 4k monitors to it - 1 over the built in HDMI and 1 via the thunderbolt HDMI adaptor.

how much ram do you have in your 13? I am curious as I may have to do some light photoshop work.

8gb of ram and 128gb of SSD (basically a base 13 model) - both have been fine for light photoshop work (not that I ever do heavy photoshop work) but I have come close to running out of storage - dropbox and a time capsule close by help, but it's something to keep in mind.

I am also planning on running linux off of virtualbox I know I wanted 16gb of ram, but I never gave drive space some thought. Would a 256gb ssd be enough?

13 2015 model can drive 4k model just fine ;)

I and my girlfriend we have two mac book pro, her is a 13' and mine is 15. I always use her notebook when I'm the road and at home :) I use the 15' from time to time, when I do some coding and I need maybe a second screen and I just want to stay on bed

i use 13.3' macbook pro and it's always enough for coding, reading and watching. i agree that it is still too heavy, in fact the air is more preferable than it.

15 if used on a desk with 4k monitor. 13 if i would not have a money and would want to carry it with me from time to time.

Why with a 4k monitor? My use case would be mostly development when I am on the go like at a coffee shop.

Because 15 is still too heavy and mostly likely will be used as a desktop replacement which you can transfer in a car and train and put it on a desk at some other place.

The 13 model is the one you carry in your backpack and use "anywhere".