I sent a message to @cybrox yesterday asking about the undocumented api and whether it will change or not in the future. it probably will change so using the mashape api would be better, but some information from the official api isn't enough (e.g anime info).
Depends on the way you are handling information, the api call for user feed provides very basic anime info and the undocumented api call provides full anime info (without reviews etc). So if you used the undocumented api to get anime information then i would use the undocumented user feed api since you don't have to deal with having 2 different formats.
To my knowledge you are only provided with the item_id and user_id and other meta data such as type, so you have to fetch the favourites to link them up.