I asked this same question a year or two ago. Some people prefer I-5, some prefer 99. Last year I started on I-5 near Bakersfield. It felt like a racetrack, with lots and lots of semis. We broke down (nothing to do with the freeway) and had to spend a few hours on the shoulder. Got towed back to Bakersfield, repaired the MH, then decided to drive up 99. I preferred 99. Didn't quite feel like the racetrack I-5 did, maybe because there are lots of towns...people slowing down to get on and off. I don't like to drive fast or feel pressured from behind to speed up. Also 99 didn't seem to have the volume of long haul semis.