LA vs. SD
LA has a better art and music scene than SD. While there are many cool, small venues in North Park and University Heights, it really doesn't compare to Los Feliz or Silver Lake.
LA's bar scene is more varied than in SD as well. Want to be a little pretentious? Go to downtown's Library Bar or The Standard's rooftop bar. There are more hipster bars in LA as there are simply more neighborhoods. For example, Glassell Park along with Highland Park are showing signs that they will be the next Los Feliz, Silverlake, or Echo Park.
So if you're looking for entertainment, LA has MANY more options.
The traffic is worse in LA. Without a doubt. It makes sense since there are commuters coming from Ventura, Riverside, and San Bernardino counties as well as LA commuters going to Orange County. The massive suburban ring around LA is huge.
In general, rents and houses are slightly more expensive in LA than in SD. Comparing North or South Park vs. Silverlake (both relatively the same neighborhoods in terms of character and feel) you immediately realize that better deals are to be had in SD.
Crime is more of a concern in LA. Okay, yes it is of course in SD and relatively speaking LA is a pretty safe large city...but it is worse in LA than in SD. Much of that is because there are large swathes of concentrated poverty in LA.
Bars are slightly more expensive in LA too.
Again it's cheaper than in LA and plus more connected with its "laid back vibe", OB and PB does boast mcmansions, to be sure, but not to the extent of similar neighborhoods in LA.
The neighborhoods are very connected in SD. Walking around in South Park, North Park, Hillcrest, or any other adjacent neighborhood, you get a sense that people are proud where they live. While this does exist in LA, I feel that it's taken to a wholly new level in SD.
SD is fairly boring at times. It lacks the variety that LA does. Part of this is simply due to poor urban planning. Downtown is now essentially a retirement suburb in which many older families from neighboring suburbs moved into luxury condos. In fact during the boom, there were more commuters living in downtown than people working in downtown. There are relatively few true urban communities in SD while there are much more in LA.
I hope this helps!