Southern CA freeways can be hit or miss, but I would think I5 is more direct.15 will take you farther out of the way and then you have to either cross on Hwy 74 or connect to Hwy 91 which will usually have traffic in the afternoon.
we are driving up from SD and for sure the 5, the 15 goes inland and you would be taking more time... plus the 15 has worse traffic than the 5 for the locals
I5 and I15 can get congested but I tend to think I15 south of Escondido can get worse. If you can be on the road by 2pm you should be ok. Closer to 3pm and you'll hit traffic. Check reports for accidents as you head out. No easy way to tell how it will be on any given day. It can take anywhere from 1.5 hours to 3+ hours.
Agree -- I15 will take you further inland so I would say to hit I5. All freeways in SoCal can stink so just be prepared regardless. And with the exception of certain freeways like 405 or 101 (which almost always stink), the grass is not really greener on another freeway.
Thanks so much everyone for all of your tips and advice! I lived in San Diego for about two years way back when...but only drove north once or twice. Wanted to be sure the 5 was the way to go! Thanks again!