According to Google Maps, it's about 45 minutes slower to go Jacob's Lake than it is to go through Vegas. So, depending upon what you want to do, either one could work.

For me, I would choose Vegas, and I don't even like Vegas. I have 6 kids, and the Jacob's Lake route gets somebody carsick every time I do it. We do Dramamine to help, but we almost always have to pull over a few times to stop somebody from puking. Also, 45 minutes longer in the car than I need to be is huge by the time I'm 6 hours down the road.