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Say you are making a user registration form, and you only want to save all the data if the user fills out all of the pages. In that case I would use SESSIONS. If you want to store the data, regardless of whether the user fills out all the pages, then I would go ahead and put it in the database right away.
In the latter case this can complicate things a bit if the user goes back and changes responses, in which case you would need to update the record rather than insert another one, but if the user stops half way through, you'll at least have some of the data stored.
As far as the amount of data that you can store in a SESSION, I'm not sure what the limit is, if there even is one. Any uploaded files are saved in the /tmp directory, so the SESSION data would only contain the path to that data. So with that said, I would not be concerned about any limits as far as SESSIONS go.