It you want to keep the extra data out of the database, then the second way may be the only way.
However, you might actually want to try it and see what is saved in the database. I thought I read something about data in hidden conditional field not being saved, remember exactly where or when I heard it. No point in doing a lot of work if it is already being taken care of. If this is the case I’m sure that ACF will not delete existing data, for example if the user actually saves the post and then changes their mind and fills in the other field.
