I believe google drive also has partially unlimited photo storage, if the photos are below a certain resolution they don't count towards your storage quota.
That's true -- photos under 2000px are free, and Google will automatically resize full-size photos as they're uploaded. Personally, I just upload my entire photo library to GDrive and G+ automatically processes them into G+ Photos. Hopefully Google rolls out unlimited full-size photo storage soon, since even I'm not sure why I pay for GDrive when I already have Amazon Prime.
In an ideal world, G+ Photos will roll out with unlimited full-size storage that deletes both smaller-resolution duplicates already in G+ Photos and also deletes full-size duplicates taken from my GDrive. I don't even mind the vendor lock-in of G+, where it's fantastically difficult to download or delete your entire library. I just want a single, organized-by-date, comprehensive location for my entire library that allows me to hoard, peruse, improve, and share my photos. G+ is so damn close to that dream already...
In an ideal world, G+ Photos will roll out with unlimited full-size storage that deletes both smaller-resolution duplicates already in G+ Photos and also deletes full-size duplicates taken from my GDrive. I don't even mind the vendor lock-in of G+, where it's fantastically difficult to download or delete your entire library. I just want a single, organized-by-date, comprehensive location for my entire library that allows me to hoard, peruse, improve, and share my photos. G+ is so damn close to that dream already...