I'm in a similar situation, and have received 3 different explanations from different Apple Store staff members at various stores. They lead to 3 very different conclusions, and obviously they can't all be correct.
The first explanation is that when you open an image in Photoshop the file will be loaded into the SSD part of a fusion drive, so all processing will be much faster than if you had the standard drive. These people said that there wouldn't be any advantage at all (for Photoshop or Lightroom work) of having an SSD drive instead of the fusion drive.
---> Their recommendation: fusion drive
A second group of staff in Apple stores said this is not the case at all! They said only the apps, system files and regularly used files are processed in the SSD part of the fusion drive, and that an image file being used by Photoshop will be loaded in RAM (so whether I have a fusion drive, an SSD or a standard HDD is irrelevant).
They said the only advantage (for Photoshop) of getting an SSD would be if all my image files were stored on it. The drive would only be used during opening/saving anyway. So according to them, if using externals for storage (as I do), there is absolutely no need to get a SSD - the "SSD part" of the fusion drive will be enough.
---> Their recommendation: (mostly) fusion drive
A third group of staff disagreed with both of the above! They said that once the app (Photoshop) was open, absolutely everything would be done in RAM (both the image file and the running of the app) - they recommended I save money and just buy the standard drive. They said that aside from file opening/saving speed, neither the SSD nor the fusion drive would make "any significant difference" compared to the standard drive, as both the image and the app will now depend solely on the speed of RAM processing.
---> Their recommendation: standard HDD drive
Obviously these 3 different views cannot all be true. If anyone could explain which of the above explanations is correct, it would be much appreciated!