Nikon's upgrade plan offers 20GB max. Google Photos is unlimited photos (compressed), or free 15GB raw/originals.
It really depends on what you want the storage for... sharing or backup. For sharing, Google is much better allowing free unlimited and some very attractive and easy sharing methods.
If it is backup, and can't see that 15 or 20GB would be enough for even the average user. With Google, you can purchase more storage at attractive rates, and sometime even earn more storage through promos from time to time. I didn't see anything in Nikon's plans that suggest you can go passed 20GB.