There is an important difference. Google is transparent about what is going on.
When I set up Google Photos I was asked: Do you want to use Google's high quality or do you want to preserve the original photos and have them count against your storage quota?
After that, you should know what you're getting into. I certainly did.
Another way Google Photos is transparent is the app will actually let you know that many of the photos on your phone have been backed up to Photos. The app will ask if you would like those backed up photos to be removed from your phone. (And it'll ask you every time since it doesn't assume that if you said "yes" once then you mean "yes" every time.) So, there's no question about where your photos went when you can't find them.
But the difference, if you read the docs, is that everything below 16 megapixels is the same for high quality and original quality. The only photos that ever get downsampled are ones above 16 megapixels.
When I set up Google Photos I was asked: Do you want to use Google's high quality or do you want to preserve the original photos and have them count against your storage quota?
After that, you should know what you're getting into. I certainly did.