What should be used to store data in GAE?
I'd suggest you split your entities in a root entity and a couple linked ones holding each some of the 150 attributes - this way, when you update one attribute, you only need to save one (or two, if the update reflects on the root entity) smaller entities to the datastore instead of a huge one.
Use memcache to prevent reads, not to store data that's going to the datastore. Memcache can be flushed and the data there can be destroyed before it hits permanent storage.
Going one step further, the grouping of attributes could reflect data that's updated together - say, if you always update street and zipcode together, it makes sense to keep them together in a single structure.
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?