15" Macbook Pro: Radeon 555 or Radeon 560?
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  


The AMD Radeon Pro 560 has twice the memory of the AMD Radeon Pro 555 (4GB vs 2GB respectively).
The AMD Radeon Pro 560 has about 10% greater Clock Speed than its rival (925 MHz vs 850 MHz respectively).
The AMD Radeon Pro 560 has around 25% lower thermal design power (TDP) (75W vs 100W respectively, lower is better).


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.