Or, as some people would phrase it: Think or swim?
It is a per-capita fact that one city has more surf shops and the other has more college graduates. But there's more to this choice than that.
One of these cities gives you reliable sun, a world-class zoo and kid-friendliness, accompanied by the occasional reminder that the 1982 film "Fast Times at Ridgemont High" was based on a true story.
The other city doesn't seem to care as much about your kids, but it will feed, entertain and edify you with such verve and sophistication that you'll barely notice the arctic breezes.
One is forever winning popularity contests, but the other is cheaper, has less crime and attracts more than twice as many overnight guests countywide.
One gave us "The Maltese Falcon," the other gave us Tony Hawk.
One is the cradle of hippiedom; the other is a major Navy port. (See stories online.) One is four times as densely occupied as the other, yet when it comes to hotels, they're almost dead even -- 226 in one city, 225 in the other.
One city has Balboa Park -- no, check that. Both have Balboa parks. But can you guess which Balboa Park once harbored a nudist colony?
One city has a winning football team, and one has the 49ers. Both have losing baseball teams (if you count up the last three seasons), and both teams play in retro-flavored downtown ballparks near water's edge.
So if you're a weekend tourist, which is better?
I hereby submit that from dusk to dawn, San Francisco is better. Well, except from November through March, when it can get cold enough to frost your Irish coffee.
I further submit that from dawn to dusk, San Diego is better. Especially if you get outdoors a lot or you're a kid.
Oh, but San Francisco will be better July 22 to 25, when 126,000 Comic-Con people will clog downtown San Diego. And San Diego will be better Sept 19 to 23, when 45,000 people gather at San Francisco's convention center for the annual Oracle OpenWorld information technology conference.
To arrive at this richly nuanced answer, I made fresh visits to each city and weighed the data with absolute objectivity and seriousness, except for the parts that I goofed around with, which follow. Your results may vary.
San Diego: To reach downtown S.D. from downtown Los Angeles, you'll probably drive. It's about 120 miles south, two hours on a great freeway day, four hours on a rotten one. Expect a round-trip gas bill of $36 if you get 20 miles per gallon and pay $3 a gallon. If you take Amtrak, it's a little less than three hours and $29 per adult each way.