I've gone both ways and I don't see how going 505 is any quicker. Mileage wise it's almost exactly the same, so for me it's faster to stay on I-5 as long as possible and get off at Castle Rock and go up to Toutle that way.
The only way 505 could be faster is if you drive well over the speed limit. I do drive back that way sometimes just for a change in scenery, but it takes about 20 minutes more than going through Castle Rock.