It's quite simple from my perspective - The 15 inch CPU is superior in all respects over the 13 inch CPU:
- 2.0GHz quad-core Intel Core i7 processor (Turbo Boost up to 3.2GHz) with 6MB shared L3 cache
- 2.6GHz dual-core Intel Core i5 processor (Turbo Boost up to 3.1GHz) with 3MB shared L3 cache
Turbo Boost engages for operations where only one core is active. Starting at a lower GHz saves energy and battery and as long as the CPU can boost to a faster top level than a competing CPU - you get the best of both worlds - fuel efficiency at idle and top speed when needed.
So, for a simple task where the computer is doing one thing only - you would expect the 13 inch to ramp one core up to 3.1 GHz whereas the 15 inch would ramp up to 3.2 GHz. Bigger is better for that task. Plus a larger L3 cache will make some switching between tasks more efficient on the 15 inch model.
Now, if you have video processing or a mixed workload where tons of compute is waiting for the CPU. You have two cores running at 2.66 or 5.32 GHz of theoretical throughput compared to 4 cores at 2.0 or 8.00 GHz of throughput. Again, no task scales perfectly, but even with 10% overhead, the quad core processor wins. It would have to be a bizarre benchmark to show the dual core i5 out perform the quad core i7 for any task that engages multiple cores.
If you want another opinion, I would recommend this nice article by Marco Arment on trying to calculate which Mac Pro CPU choice works for various budgets and tasks:
In it, he doesn't explain why he feels the 15 inch MacBook Pro is so far superior to the 13 inch MacBook Pro and simply states it as fact. I generally agree with that due experience and seeing benchmarks/reviews I trust. My feeling is the current 15 inch logic board and supporting components are faster and of better specifications than what is put into the 13 inch model. If you can swing the cost and live with (or prefer) the larger physical size, get the 15 inch model. It's an excellent workstation in a portable form factor. For many tasks it benchmarks as fast as an iMac and even the new Mac Pro.
Personal note: The base $1299 13 inch model is no slouch - I work from one almost exclusively the last year and I'm never really waiting for it in practice. If you need to pinch pennies on the CPU, I'd con side any of the retina 13 inch models if it means you can afford AppleCare, an external monitor plus backup drive(s) and whatever software you need to be productive. Rather than invest in tons of extra CPU cycles that you may not really use or need to work day in and day out.
Best Answer
Introduction
I think we need to agree that both of these graphics cards are the best they have ever been in a MacBook Pro. Naturally, the type of card you require is heavily dependent on the work you plan to do with the MacBook Pro (you didn't mention it). If you don't yet own one of the devices and are simply doing some research before you buy (as one should), you may find the next generation (releasing around June 2018) may be a better alternative - ahh, when will the time ever be right!?
Notable Differences
Reports
There are GPU benchmarking sites like GPUBoss and Notebook Check if you are interested in further reads. Otherwise the difference is next to none.