I've tried Google Photos a few times and went back to Apple every time. First and important, I consider them both lousy apps. They are terribly deficient or executed poorly in many areas I deem important (album composition). Having said that:
- As above GP is available on iOS.
- If cloud storage is important to you, Google. However, realize Google can/will scrub your images and can make them available on their image search. The last time I used GP I created some albums, the images were location named and I shared a link to the album. Later I did an image search for a specific location and low and behold one of my shots was in the results. That's Google. If that's ok with you, go for it.
- If you compile albums and order/sort images in any order other than date placed on line. GP works fine. If you want control over the order images show in an album, since iOS 10 images in Apple Photos can be sorted by tap/hold/move. For me, what I want as my Mac/Lightroom is my primary image management/editing tool.
- If you edit images, Apple all the way. The editor in GP can do some really weird stuff to color hue, luminescence and saturation. I also noticed severe compression after editing in GP. Maybe a setting I missed.
- Neither are committed to Photos. They just want your business and offer a rather poor app to check off an item on their "must have" list. Apple has gone from MobileMe to iPhoto, to Journals, to Photos. All not seemless transitions and all poorly executed. Google went from Picasa to Photos, a lousy transition for Picasa users. If your Photos mean anything to you, having local backup and a robust image browser that's been around a while makes sense.