QUOTE(shockk @ Dec 23 2015, 12:18 AM)
Having personally used the first gen 15" Retina for 2 years, then switching to the 13" Retina for about 1 year, and now finally back to a 15" Retina again, I have to say that screen size does really matter. It's just perfectly sized for side-by-side app usage, photo-editing, and graphics creation. Surfing the web is also a better experience.
On the portability front, yes, the 13" Retina is undoubtedly the best everyday-carry winner here. You basically will not feel it much. However, I also carry the 15" Retina a lot everyday on a messenger-style sling bag, and I don't really feel it too since it's only 2.04 kg. I normally do not bring the charger with me since it is rated for 9-hours wireless web and the performance does not disappoint. The 13" Retina is rated for 10-hours so technically it should have even better battery life than the 15" Retina.
The last bit that I like more on the 15" Retina is its dual fan design. The 13" Retina used to have a dual mini-fan design, but it was reengineered to a single large-fan design. Each design has their own pros and cons, but its just a personal preference for me.
That's all I can that I can think for now apart from the already-mentioned differences in CPU cores and graphics hardware.
Happy deciding.

I personally had owned the latest mid 2015 15-inch for 8 months and I sold it away after that.
Pros about 15-inch
More screen real estate for video editing
Greater and faster processing power (only MBP with i7 quard core) for video rendering
More default ram size (16GB)
Cons about 15-inch
Heavy - 2.04kg (despite that it is around half a kg heavier than 13-inch) but it does feel a lot more heavier. Probably due to the fact that the weight of 13-inch is already at the max before one considered it as heavy, so anything above it, it is real heavy.
Bulkier - due to the bigger screen, the form factor is a weak point compare to 13-inch.
Power adapter is heavy, and it is a lot heavier, it is 2x the weight of the power adapter for 13-inch model.
Battery life - it is not excellent and it is not really near to all day of usage as claimed. If u use it as work, it definitely cannot last for the entire day without charging. So power adapter unlikely that something can be forgotten at home unless it is not for work purpose but just on the go for some casual usage that is meant for full day usage, it is still ok. For web surfing it may be able to reach 8 to 9 hrs on OS-X but if it is to do slightly more intensive tasks, realistically it is at most 3 hrs plus for video editing etc. if ur model is the one with discreet graphic card, it is even more battery drain.
Cost against perceived value is low, given the price for 13-inch and the tech it comes with, it is a value buy, but when it comes to 15-inch, it has a great drop in value due to expensive price of 15-inch.
Heat - 15-inch is a power haus with desktop power, but it is a lot hotter during operation.
Fan operation and fan noise - it is noticed that the fan is on almost most of the time even on normal tasks. Thus fan noise could be a irritating for this laptop.
PPI - it is lower than 13-inch.
As an ex-owner for the 15-inch, I will only recommend the 15-inch to those who really needs the extra power such as for video editing or rendering, else for normal users who only uses word processing, web surfing, video watching etc, it is a waste to get this machine, unless u really want to the bigger screen. Else it is a waste of money.
I hardly even able to use more than 20% of the cpu and ram power of the laptop most of the time.