My opinion is a bit different. I think there are some pretty long grades on I-5 north of Redding but it is freeway and many of the climbs have added an extra lane as a passing lane or truck lane. It'll be hot so make sure your cooling system is in good shape.
A little info I picked up off the internet:
While the town of Weed has an elevation of 3,467 feet (1,057 m), a section of U.S. Route 97 in California is above 5,000 feet (1,500 m) in elevation with two significant summits. At mile 19 the Grass Lake Summit has an elevation of 5,101 feet (1,555 m). There is a rest area with restrooms at Grass Lake about a half-mile south of the summit.
At mile 30 the Mount Hebron Summit has an elevation of 5,202 feet (1,586 m). The small community of Mount Hebron
follows the descent from the summit, but is located a couple miles east of the highway. The next community located directly on the highway is unincorporated town of Macdoel
. Here's site on hwy 97. Weather and web cams courtesy of Medford Fuel / Pacific Pride
That said both I-5 & 97 are good roads for us travelers. Enjoy the trip.