What should be used to store data in GAE?
First of all you should find out how you are going to use your data. Which queries are you planning to make? What is the size of your entities?
Datastore is very different from relational databases. That is It doesn't really matter how many properties are changed, because there is no way to update a property on its own. You can only save entity as a whole. I don't know about your use case, but there should probably be a better approach for structuring your data, than having ~150 properties for a single entity. What is your use case?
Also having too much properties for an entity may lead to index explosion, or to slow down datastore writes.
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?