Difference between photostream and icloud photos
Each of them comes with their own advantages.
Advantages of Photo Stream:


Does not use your iCloud storage
Only stores your most recent 1000 photos or the last 30 days of images
Does not upload and sync videos


On the other hand, here is the advantages of iCloud Photo Library:


Stores all your photos and personal video
Uses your iCloud storage
Stores full-resolution photos


If you want to see a good article about the whole difference, you can check this link.