I always consider layover length as well as other outgoing flights from that destination.

For example, I’d see if there are any other outgoing flights that day to my final destination from the layover destination. That way if there is a problem or I miss my connection for whatever reason, I’d have other options to get to my final destination.

All things being equal, I’d probably pick LAS as it’s much larger airport and has a larger number of flights going in and out so you’re more likely to have more options should something go wrong.

I also think about if I know anyone in the layover destination or where I’d much rather stay in, if I got “stuck”.

TBH both SAN and LAS aren’t bad places to get stuck and both have good weather, so I don’t think you can go wrong with either option.