Anyone that has lived there wanna give me a hint?
Carlsbad is a small-town atmosphere, at least by California standards, fairly wealthy, not a real homeless population. Great antique shops, independent stores, easy to park and walk around most of the whole thing. Good beaches, of course.
San Diego is a huge city with an amazing variety of stuff to see and do, I_ve lived here for 45 years, and they seem to invent things to do and places to see faster than you can experience them.
If you are driving from the North, C_bad is closer. Its harder to fly to. The San Diego airport is right in town. The weather will be nice at both places.
If you are going to live and work, then Carlsbad is fairly limited without a long commute. I trust you are doing research other than asking a question here...