This is something I am currently trying to do too. So far I am having success with using Clay.io to store data remotely, and then using that data to store locally at the same time via webstorage, so a player can play offline and all working in CocoonJS. It's still early days but I am pushing all my data into an array that I send to Clay.io as json.
I would recommend CocoonJS just because it's better performance atm and Clay.io because it has an awesome plugin and features.
One thing to note if you use Clay.io you need to manually include some files in the exported zip for CocoonJS Every Time (their case sensitive too) :
You can get those files from the clay plugin directory in C2 (once you've installed it of course).