The Detroit Lake loop is just about 300 miles of entirely paved highway. I prefer to head out towards Estacade first so I've got the maximum amount of daylight when alongside the Clackamas River. It tends to get dark there early due to the steep hillsides bordering the road. The closer you get to the midway point (Detroit Lake) the better the road gets.
I can't say much in favor of Highway 20 headed westbound back in towards Salem. It's a bit of a necessary evil to complete the loop. Its too heavily trafficked and long/straight for my tastes. You know me: too much a fan of the alpine twisties to bother with major highways for too long. And for those on that bandwagon might I suggest taking 99E northbound back into Portland instead of I-5. It's far more rural, and offers a great view of the mountains you were just rallying through without all the semi-trucks and commuters. The whole loop is totally do-able before sun-down if you leave right after work and don't hit any traffic on your way out of town. I do recommend heading through Estacada first to get you through the woods before the deer start coming out for dinner.