They can be equally fast. It depends on the time of departure and timing of connections.Note that the farm can be reached on foot from "Lavender Batake Station" along the train line, which connects Furano with Asahikawa.
You can look up connections by entering "Sapporo" and "Lavender Batake" on the following website:
http://www.jorudan.co.jp/english/norikae/
Taking the bus from Furano Station would be possible, too.