What should be used to store data in GAE?
The datastore is your database. Memcache is to store data that's fetched from the datastore and kept temporarily in memory to avoid too many calls back to the database. You should first design your app around the datastore and then use memcache to improve performance.
Depending on your programming language of choice (java, python, go) there are many tools out there to help you map objects in your app to the datastore and to use memcache effectively.
My web application will have ~150 fields and when value is changed in any field (at least one), I should save changed value. 
How should I store such values with GAE? Should I save them directly in datastore? Should I use memcache temporarily and then save all values at once in datastore? Or, some other approach should be followed?