I am also from Vegas, but I have drove the I 5, I think that highway is really really busy when I drove from Central Cali to Disney. It was packed at any time of day but midnight.
I would say it would be good to go on the other but someone else might have more insight since I am not sure how it is, but I know I5 is busy.
We drive down to Anaheim from Lake Tahoe every October (and we are driving down tomorrow ). We used to take I5 because we had to drop my dog off in Sacramento, but the last couple of years we have taken 395. I think the distance and time is pretty close - maybe 15 or 20 minutes more one way or the other. So it really depends on what you prefer. You can go pretty fast on I-5 (although there have been threads about construction slowing things down but I am not sure where) - it is very straight and there is not much to see. But there are lots of places to stop for food and gas.
395 on the other hand has some very pretty stretches and some very boring stretches. Not as many places to stop for food/gas but a lot less traffic in general. Bishop, Lone Pine, Big Pine all have various fast food places. Once you get further south (maybe 60 miles north of I15) you start hitting truck stop type intersections with gas and mini-marts.
We choose 395 mostly because it means we avoid I5 altogether in LA - and I5 can be backed up at any time of the day or night anywhere through the LA area in my experience. And the stretch from Minden to Lone Pine can be much prettier than anywhere on I5.