From the website www.macleanclan.com:

MacLean of Duart Dress Tartan

MacLean of Duart tartan is the most common tartan used my members of the Clan MacLean. It is also the basis for the Royal Stuart Tartan. Since kilts are usually only worn on semi-formal to formal occasions, this tartan is the primary tartan used today.

MacLean of Duart Hunting Tartan

The Hunting tartan of the Clan Maclean is the oldest recorded tartan in Scotland. A discription is included on a charter for lands of Nerrabolsadh on Islay in 1587. It is this, hunting, tartan usually worn for more casual occasions.
Not sure how authoritative the site is, but I've seen the same information repeated elsewhere.