We prefer using Hwy-99, it's all freeway these days with no 'stops'...
It's true if you travel through the few larger cities during normal morning or evening commute times, you'll see additional traffic, but nothing like what you'll find in the Los Angeles basin!
Towing a trailer, and moving right around the 55 MPH speed limit in Calif (with trailers), we find Hwy-99 a more enjoyable route - many more places to refuel both us and the Excursion!
Interestingly, as I ran the mileages (Streets & Trips) from our home just north of Stockton, it's within one mile of being the same, using either Hwy-99 or I-5 to down south of Bakersfield where the two routes join up...so towing either route would be no advantage - however if you're in non-towing situation, you would be able to hit 70 mph on I-5 for a faster passage...
Also, it's within one mile of being the same distance if traveling from Stockton, via Hwy-99 - Bakersfield - Hwy-58 East, then South at Kramer Junction-Hwy-395, I-15, I-215 to San Bernardino and I-10, on to Palm Springs...
OR...using I-5 south all the way south, then East on I-210 to Riverside and on to Palm Springs...same distance...
Depending on the time of day, the I-210 route could cause some delays due to the crowed traffic conditions down there...
I far prefer the former route through Bakersfield, East to the desert, etc., when towing, we're not in any big hurry anyway...yes, there are trucks on the road, but that's everywhere here in California on the major highways...
Hwy-99 does have some roadway sections that are on the 'rough' side, but hey, you're in California and many of our roads have gone to crap in recent years!!!! The State takes in BILLIONS each year in vehicle and fuel tax fees, but likes to spend much that money elsewhere instead of improving our much used roadways - but that's a story for another time... :-[