I’ve used both, and I am in a similar situation. I chose to drop PDF Expert.
For full screen and for quickly viewing and searching PDFs, I find Preview more than sufficient. It has flawless scrolling for PDFs and full screen is better than PDF Expert.
I use Acrobat when I need to process and edit PDFs, PDF Expert has only a fraction of the features and is much less capable. I also have a free subscription, so I don’t see any reason to pay another $50 a year, especially as I much prefer to mark up PDFs with GoodNotes on iOS. Acrobat offers best in class OCR, Microsoft Office integration and better cross platform compatibility, especially with forms. It’s also the most powerful tool for optimising and exporting PDFs.
I use Preview as the default for both images and PDFs. When I need to edit something I go into an editor. This way, I get the best of both worlds - fast viewing and professional grade editing.