Performance: 13" vs 15"

Definitely. The quad core CPU and stock 16GB RAM alone will make a huge difference. Plus the dGPU will be much better for running external displays.

 

I doubt it'all make much difference for coding (apart from screen size) but graphical design will of course benefit hugely from the quadcore and dGPU depending on the apps you use it could be more than 2x as quick in some use cases.

The 5k screen is a beast to run and it's amazing the 13 inch does it at all with its iGPU the 15 inch will be a lot better but you may be better off with the 460 dGPU upgrade it really is a much better GPU and will make a difference over the next few years if not now.

 

Programming, specially compiling, is generally much more dependent on the speed of memory and in that regard the 13" and 15" models are identical, so there shouldn't really be any tangible performance difference between the two in that area.

As for graphical design, it's a rather large field and like programming, it's a rather memory intensive field so the real world gains may not be as big as the theoretical difference in CPU and GPU performance. Thus I'm going to have to say that you're going to have to say more about what you're doing in the field of graphical design before I can make an assessment on it's worth it to upgrade to a 15" machine.

 

Running VMs and high-res monitors (both are essential for programming) is where the 15-inch really shines, imo. I don't have a lot of experience with graphic design but the dGPU can't hurt in that respect.

I actually upgraded to a 15 inch (455) after using the 13 inch w/touchbar for 2 weeks. I just couldn't get over paying almost 3K for a computer I wasn't 100% happy with in terms of performance (13 inch stuttered noticeably when under heavy load and connected to an external monitor). The $500 difference to upgrade to the 15 inch was well worth it for me, just for the smoother experience - the extra screen space is a nice added bonus.

Hope this helps.