If you go to JC, you will be basically getting all your GE out of the way so that when you transfer to a planning program you will not have to worry about math, stats, liberal arts, sciences, etc.. You may still have support classes to knock out, and of course your electives. But in California you will be getting your IGETC, which certifies that you have all your lower division GE and are ready to transfer to a UC or CSU.
This is what I did: Went to a CA community college in Northern CA, got my IGETC certification, and got an AA in Liberal Arts, since it only required a couple more units or something. In the meantime I applied to Cal Poly SLO, UC Davis, UC Berkeley, and UC Santa Barbara. After receiving my acceptances, I chose Cal Poly SLO and did the BSCRP program, which in hindsight was probably the best choice, but who knows. It was definitely a better choice than UC Davis or UCSB, based on input I've got from others I know how did their undergrads at those places in the community development program (Davis) and the Bren School for environmental studies (UCSB). SLO is also a great place to live. After my BSCRP I went to work for a private planning firm in town, and knocked out my MPP also at Poly. I am now moving to Washington DC to do planning for the federal government. A long way from my blue-collar, sleepy hometown in an obscure part of northern CA.
I
highly, highly recommend doing 2 years at community college first. The advantages are overwhelming: 1) Cheap. I didn't pay a dime, and in fact was effectively paid to go to school with federal and state aid. Even if I had to pay out of pocket, fees minimal; 2) I could figure out what I wanted to do first, and where I might like to go; 3) I ended up being more competitive of an applicant since I had that time to boost my GPA with easier GE classes; in addition, most colleges favor transfer applicants, so there's no downside to this (see here for proof:
http://www.ess.calpoly.edu/_admiss/P..._090409pdf.pdf); 4) I didn't have to make any big decisions about moving somewhere else in the state, which would also have been way more expensive; 5) It was a nice introduction to college in a smaller, less competitive academic environment. For me, who had been out of school for 6 years prior out in the workforce, this was nice. I never took the SAT in high school and never intended to go to college in the first place.
I really can't see many advantages in going to a university for the full 4 years. You'll certainly probably have access to more social aspects of a university, but will also probably have harder classes and pay at least 10-20 times as much in tuition, not counting living expenses if you have to move. You'll have to declare your major as a freshman if you go to Cal Poly SLO, so that is another big reason to transfer in, since you should not trust yourself to know if you will be a planner yet. I feel that most kids should just go to college first, learn about all sorts of stuff, be exposed to things, and then figure it out gradually. Doing 2 years at JC would accommodate that.
EDIT: I'd just like to add that I don't think you should alter your choices of where to apply based on what you think your chances are according to your SAT or GPA. There are more criteria used than grades. You should just figure out where you want to go regardless of how hard you think it is to get in and then make the best case for yourself while applying. Good luck!