It really really depends on what you major in or how your personality is like. I don’t know much about Sonoma, but If you want a general opinion in terms of location, small class size and focused programs, I would say Humboldt. I have a couple of friends going there and they are generally happy about choosing Humboldt. It is truly BEAUTIFUL up in Arcata where the campus is located. Humboldt is a bit like Stanford in terms of location, because it is a secluded school surrounded by great scenery and nature. Granted, there is not much to do in terms of city life, but students fulfill that crave by taking part in many parties. Also, if you plan on majoring in the life sciences such as biology, marine biology, botany, environmental studies etc., this is the school for you out of the three option, In my opinion.

Humboldt also has good study abroad options. Also, the class sizes at Humboldt are fairly small in general (although still depends on what you major in), which allows for better learning, friendly instructors, and a feeling of community among your fellow students. That feeling of community and friendliness is something very valuable and underrated. I know of many students that were robbed of this when they got to some of those large student body schools where you honestly feel like a nobody, and became quite dissatisfied with their experience no matter how prestigious of a school they attend. Prestigious AND large student body is definitely a lethal mix, only to be taken by a highly social and leader minded student. You have to know who you are, and be strong willed. With the type of student that attend these schools will hand you an identity, if you don’t know your own. And that is not a good thing.

As for Northridge, to my experience, it is an average school with taking some classes over there while I was in high school. This school will suit you if you are an active student with an attraction to city life. Depending on what your scene is, whether city slicker or nature fanatic, you may or may not like the location. Also, the location of the school is inconvenient if you don’t live near or on campus, since you have to deal with the heavy SF Valley traffic every day. Parking is a mess. But, if you are a city person, you might like CSUN better than Humboldt.
However, I’ve met lots of people from there who were happy with their experience at CSUN, so really, it is all a matter of taste, personality, and what you major in. From what I hear, CSUN is excellent for business/accounting majors, MBA candidates, or prospective educators looking to get a master’s or teaching credentials.

Some of the factors I would consider:
-Whether you are a city person or not. For example, do you enjoy or does your major allow time for you to partake in activities outside of school such as nightlife, shopping, etc.?
-What is your personality like? Introvert or extrovert? Do you prefer bigger class sizes or small class sizes? As for student body size, in terms of analogy, would you prefer being the fish in a small pond, or the fish in a big pond? Inner city schools generally have impacted classes, and large student bodies, while outter city schools, have smaller student bodies.
-What is your major? If you plan on going towards something occupational like business or law, I would suggest CSUN, not only because of the great business curriculum there, but you can develop better connections and a better business network because the campus is located in Los Angeles, which is where most serious business is conducted.
-Do you plan on living on or away from campus?
-Consider your planned cost of living. For instance, it might be more expensive to attend CSUN because the cost of living in L.A is considerably more than Sonoma or Humboldt.

I am sure I am leaving many other great questions, but I would say these are the basic questions one must ask oneself before selective a campus to attend. I would also do research: read student forums, go on the campus’s website, go on collegeboard.com and see the statistics for each campus, request brochures from each campus, communicate with students in your major at each of these schools via forums, alumni, etc.
I also STRONGLY encourage you to visit the campus. Not only is it vital to do so before you actually enroll, but this might be the big huge tie breaker in your decision.

I hope I provided you with some useful information to contribute to your decision-making process.
Good luck! =)