It varies. When I was trying to keep track of configurations used in December (in preparation for my RDU-LHR flight at the end of the month), there was usually a 4-to-3 mix between Zodiac and Super Diamond configurations every week. Some weeks there were more Zodiacs than Super Diamonds, other weeks it was the other way around. The aircraft (and config) doesn't get assigned until 48 hours before the flight.