I am thinking of archiving my past photos in digital format. I am pondering over the quality of using a slide adapter over a film scanner. Which one of them has a better quality?
Short answer: film scanner.
In my limited experience with film scanning the dedicated film scanner always wins in terms of quality (sharpness, dynamic range, resolution, etc.) over slide adaptors on a flatbed scanner.
If you want the highest quality, you can always go for drum scanners (if you can afford... )