5 south to the 22 to the 55 to the 91 to the 15, about 4 hours driving time, you really would not save any time flying, as long as you leave in the morning it will be an easy drive.
Lived in Vegs for a bit...Will be nice when the Bullet Train between LA and Las Vegas is up and running!
Flying that distance is annoying and driving at the wrong time can be miserable!
To the OP...depends on when you wanna go...random Tuesday morning...driving is just as quick as flying door to door...need to go on a Friday evening into Vegas?...fly.
Best driving route to get to I-15? Use Waze at your time of departure...traffic can be hit or miss at any given pint at any given time.