The answer is simple, if you have a lot of data handling to be done, go for Core Data.
If you want to manually write SQL queries to store and retrieve data then go for SQL with FMDB wrapper.
FMDB is good when you have less data to be handled and if you prefer to write your own data fetching code.
Let's take your example of inventory system for products. In Core Data, you would create an entity called `Product. And Core Data would be responsible for handling all data saving and fetching of a particular entity. And whenever you add a new attribute to the "product", Core Data would handle it for you.
Now, the ramifications if you were to use FMDB, then you would have to write code to save & retrieve from the database. And, whenever you will make changes to the database structure, then you will have to manually add/remove code for the schema changes.
Here are a few good links for Core Data:
http://www.raywenderlich.com/934/core-data-on-ios-5-tutorial-getting-started
http://nachbaur.com/blog/smarter-core-data
Good Luck