San Marcos is actually quite the up-and-comer city. It is well planned, the city has been extremely fiscally responsible, and the schools all have improved quite significantly over the last few years. There are already several new and attractive shopping areas. The city is finally breaking grounds on a long planned downtown area after clearing out a huge previously low income Latino area. of course the gradually growing CSUSM and the growing Kaiser hub there all will make it the center of north county.

I would say if looking at a place to invest in the north county, it would probably offer the most bang for your buck and higher chance of appreciation.