Dear Nick,
I grow both DF and EL in my woodland and am currently framing using both materials.
They are quite different and you need to use them appropriately in your framing plans.
If limited to one timber I would choose larch.
I recently saw some traditional timber framed barns in Southern England (circe 1750) which were framed in large section larch and this really looked the cats whiskers. Its easier to get long straight and also swept curved sections from larch with minimal sized side branching i.e. small knots. DF tends to have much larger side branching and can grow too fast near the but end so waste the first 6 feet or so.
If converted and stacked properly using slow air drying the Douglas will prove to be much more stable than the larch which seems to be a little more prone to twist.
Both materials are very much under rated here in the UK and can be picked up quite cheeply from the woods where there is no ready market for this material because of low cost imports of Latvian softwoods.
Please email me with any specific questions.
Regards
Ken Hume