I use OneDrive, Google Photos and Flickr.
I think Google Photos is great if you are just backing up casual shots. They store up to 16MP photos or 1080p videos for free. If your photos or videos go above that they compress it down. Also even if your photos do not reach that limit they still compress them slightly. In most cases it's hard to tell the difference between the original and the one Google compresses, but it is noticeable if you look closely. To me it seems like Google Photos takes away some colors to make the photo smaller. What this means is that no matter what you upload if you do not choose to upload the "original" you lose your original photo once you delete it from your phone if you haven't uploaded it somewhere else. OneDrive stores your original file as far as I know.
If you choose to upload the "original" then you forgo the free unlimited storage and then Google Photos storage comes with the same limitation as OneDrive where you need to pay for storage.
Another downside of Google Photos free unlimited is of course it's free. If Google one day chooses not to offer it for free then you have to figure out a what to do next. However, as we know Microsoft isn't worried about shuttering paid services or rolling back "free" services either. Either way it's a risk.
If you go with paid services OneDrive I think offers better bang for your buck. With Office365 you get 1TB each for 5 Microsoft accounts. That's virtually unlimited for most users. But, if you are just interested in photos and don't care about your photos being compressed I think Google Photos is great.
As for the services themselves I find Google Photos to be a lot more consumer friendly. OneDrive at one time was better, but in usual Microsoft fashion they pushed ahead with some nice features only to give up and let Google surpass them.
Some features Google Photos has that I like:
None of the above are really must have features, but I find they make Google Photos more "fun" to use. It's nice to see some photos after a while put in different ways as most of the stuff I take I don't even look at again. If you don't like these things I think you can turn them off in the settings (not sure).
OneDrive has some of these features, but to me OneDrive's implementation seems a lot less intelligent compared to what Google Photos does.
Both OneDrive and Google Photos let you search your photos. So a search for "dogs" will bring up pictures you took of dogs. I feel Google Photos does a better job at identifying pictures. One Drive however does a better job identifying text in pictures. Google Photos as far as I know one ups OneDrive in that it also recognizes people's faces and can group the same person. If you tag that person you can then search for something like "grandma" or "me". I don't think OneDrive does this yet, but I've heard it might one day.
As for device support. I hate that Google doesn't support Windows with apps. You can of course just use photos.google.com in the browser, but that sure didn't help when used a Windows phone. OneDrive on the other hand has worked well for me on Windows in the browser, in the One Drive app (on Android too) and in Photos. I hand it to Microsoft for making their services work even if you are not using Windows, although I do wish Microsoft would make their services better on their own platform.