I5 is the fastest route. Pretty boring drive but consistent. CA99 you will come across lots of traffic coming in and out and there is more to "see" (there are actually cities off the highway). You will see lots of trucks on CA99 but they are probably not driving long haul or they would have taken the I5.
Since you are going to Sacramento I probably would go with CA99. At least it will keep you a little more engaged driving.
But it is up to you. Neither is going to be a scenic drive but if the time difference is not much the 99 would be slightly more interesting.