We've taken that trip dozens of times, and prefer highway 36 out of Red Bluff. It's a good road (rare in CA), and not especially curvy, except in one very brief spot where the road is changing canyons. It's best to avoid driving east from Red Bluff up the mountain in the afternoon in triple digits. It is a long, steady climb, with about a 5,000 foot change in elevation from Red Bluff. We nearly always drive I-5 first to Red Bluff, spend the night there, then make the climb out of Red Bluff in cool of the morning. Returning home, we use the same route, but since it's all downhill, heat is not an issue, so no need to stop in Red Bluff.