Never been to either one. Any suggestions on which is better for a 3 day visit?

Sf

One nice thing about SD is the airport is next to downtown, can get a lot done on relatively short weekend trip.

Depends what you want to do. SF has better food and nightlife while SD has better outdoor stuff. Personally, I like San Diego more than San Fran.

For only three days, I'd say San Diego is better. You could spend a full week in San Francisco and still have things to do.

For me:
With kids-San Diego
Without kids-SF

San fran is a great 3 day trip but the bums and crackheads seem to have multiplied lately..
There is a lot of cool sites/things to do in SF where SD is more of a beach oriented town with some great nightlife.

All things equal I would go SF.
If weather is a factor I would go SD.

For 3 days and never been to either you really cant go wrong.

You are going to want to see both one day so pick one now and do the other one later..

SD 100x.

What kind of things do you like to do? What is your budget?

Just got back from San Francisco with the family. Biked the Golden Gate Bridge to Sausilito, Fisherman's Warf, ferry to Alkatraz, amazing food, Chinatown, Matisse exhibit at the SFMOMA, and a Warriors game at Oracle Arena. This itinerary might not appeal to everybody but it was great for us.

dc that sounds like a great trip. How many days were you in sf?

Arrived afternoon of Saturday, March 11th; returned early Wednesday, March 15th.

Tartine Manufacturing (Mission District): amazing food.
Ferry Building is really cool.
Mr. Holmes Bakery has amazing cruffins.

Also, you don't need a car in the city. But it is easy to rent a car for the day if you want to drive up to hike among the giant redwood trees in the Miur Woods (Uber might take you there, but cellphone service is spotty and you may not be able to get a ride back).