Given you've played caverna extensively you'll have no trouble with any of the cards in agricola, so you might as well play with all three decks mixed up. To be honest there isn't that much of a complexity difference between decks, and you'll get a feel for which are powerful quite quickly.
It is not so much complexity as that they were balanced assuming you only use one of the decks. Admittedly, most folks don't do that. But if someone wants the best experience possible as a starting point, he or she should start with how the game was designed.