Disclaimer: While I have used Adobe products (available to me at work), my experience with them is limited. I've never had to do significant amounts of image editing. I've played with my copy of Affinity Photo but not used it in depth, and I actually use Affinity Designer.
The short version: Affinity products:
fewer features (especially "non-core" features),
better interface (in my opinion).
Photoshop has become a sort of Swiss army knife of "use it for everything image-related" and, while Affinity Photo has features for digital painting, Affinity Photo seems to have more of a "built for photographers" feel to it (I'm not a photographer, which is why I use it less than Designer).
Affinity Designer is more of a "create from scratch" program, and feels like the child of a
vector software with a
paint program, with the ability to use
raster brushes "onto" (clipped by) vector shapes. AD almost seems a little as if Adobe's Project Gemini came out 4 years early for Mac, then Windows, then iOS.
While Illustrator has always felt like "built for designers" to me, Affinity Designer feels (somewhat ironically, given the names) more like "built for digital artists" (in spite of a plethora of
design features).
[Incidentally, the SVG output from Affinity Designer opens fine in Moho]
Trial versions are available, but I believe they only have about 10 days duration:
https://affinity.serif.com/en-gb/signup/trial/photo/ (feel free to replace en-gb with en-us for US English, or de for German)
https://affinity.serif.com/en-gb/signup/trial/designer/
Note: the trial versions are a separate download version, they do not update to a full version, although
saved projects should open in the full version.
The print-only workbooks, Affinity Photo Workbook and Affinity Designer Workbook are worthwhile investments, high-quality project-based books (note:
not user manuals, the in-built help is probably better for that), available in English and German.
Credit: several links above go to Frankentoon tutorials for examples. No affiliation, except as a fan and occasional customer.