Given those choices, I would go on I-5. 99 goes through a lot of small towns/cities with lots of on/off ramps. I-5 is very boring, but you can pretty much maintain a decent speed. Last time we went through that route, the traffic was pretty heavy in Modesto and Sacramento with lots of construction going on. Have no idea if that was completed or not. Haven't been up 99 in quite a few years....but it was in need of repair in several places and really rattled the motor home.
I would stay on I-5.
Ron