Not much help when it comes to the City.
I haven't been to either LA or SD as I so much more prefer spending time at the natural wonders than the uraban sprawl. Both would be viable options and it's difficult to avoid LA on the way to San Diego when travelling down the coast, although I must say personally I find SD a nicer proposition. Having said that, and as you are probably aware by now, I'm not a 'City person' [although I loved San Fran] and I would be looking more at time spent in places like Monument valley and even Zion and Bryce canyon in Souther Utah, Zion being just 3 hours from Vegas.
Much will come down to how much time you want to spend 'where'. You might find that completing the loop and end in the same City in which you started from, might be favourable when it comes to costs of flights and car rentals. You could see which City in the 'Loop' offers the best combined rates and start and finish there.
Depends on what you want to do.
Like Dave, I am not much of a city person, when it comes to travelling. But I have been to LA multiple times, and to San Diego.
In the end it all comes down to what you want to do. All cities have many tourist traps, and your dollars can dwindle very quickly. When I am in a city, I like to walk around, get a feel of the atmosphere, see where people live, what types of things are available in the average store and speak with the locals. When I purchase food, it is in the local supermarket, where one nearly always gets a chance to speak to the person in front or behind in the queue. I never choose a checkout which is empty.
Although I have been to both cities, I have not been to any of the so-called tourist attractions... zoos, hollywood, disney, or anything else.
So as I said, which city you choose, comes down very much as to what you want to do. Of the two you mentioned, I'd probably choose San Diego. Just that my flight always comes into, and leaves from LA.
Lifey