Simon, both will eventually turn gray. The cedar will be a nice silvery gray, the doug fir will become, well, kinda ugly gray.
The doug fir will also have a tendancy to attract mold spores if left unsealed and get some blotchy black/green patches to it.
For the cedar, the best I can recommend is to let several coats of the rosewood oil soak in and dry thoroughly between coats (about a day each) and then use a spar varnish (I think it's called marine varnish there?). A few good coats of it does wonders because of its UV protection.
The problem is the doug fir. Spar varnish remains a very flexible finish. This can become tacky/sticky in direct sun. While it will help the color, people sitting on it may stick to it on a warm day!
Again, the rosewood oil will provide some good UV protection for the doug fir, but the film finish is the issue. I would make up a sample board of doug fir and put a few coats of spar varnish on it. Let it fully cure for about a week out of the weather. Then place it outside on a hot day and see if it is acceptable.
I would also contact Bob Smalser, he's a member here though he doesn't visit often. A PM might alert him to a question. He might know if you can alter the properties of the spar varnish to provide a harder finish (Terry Smart of Chestnut Products may also know come to think of it) and still provide UV protection.