Better in what ways? Where are you going to afterwards? If you land at LAX, at least, you'd deal with Immigration and Customs in one terminal. The others are irrelevant. I've never had any problem with I&C at LAX, but haven't been through that in the UA terminal. Of course, one's expeience can vary widely by the particular people you end up dealing with, sweeping generalizations aside (even though they are often based on anecdotal evidence).

Where are you going to for your onward flight? LAX would, in general, have more flights, so if you somehow miss yuor conneciton you may have more options for another flight (assuming your itinerary was purchased all together, UA would put you on the next flight with available seats, but this may not be the very next flight). Should be a short walk at most to gt to your next flight (though, of course, you'd have to go through security again). If you had to switch airlines, LAX's size (including distance between terminals) could be a factor to consider, but I believe UA uses just two adjacent terminals.

I personally prefer larger aircraft, so what type of plane I'd end up for my onward flight, so that would be a factor for me, though a smaller one than cost and time I have between flights. I've seen airline Web sites that give options for ridiculously short connection times. Even if they will have responsibility to get you to your ultimate desination, you don't want this to involve a hassle (e.g., super long wait). I assume you've found that for your travel dates the fares are the same whether you go through LAX or SFO. If everything else is equal, I'd probably opt for SFO if for no other reason than you may get a view of SF as you land/take off and you can get some good sourdough bread in the airport.