my vote says go to the 4-year. I think you miss out on some of the college experience if you go the JC xfer route.
For one thing, frosh year everyone is in the same boat of knowing few people on campus are are very open to meeting new friends. After 2 years together they have met people and have a circle of friends; a xfer who is not outgoing may face more problems in making new friends than they would have frosh year.
Secondly, by the start of junior year the students who have been there from the start are old pro's at how ways things work at the school, the stuff you just learn from experience. They have friends they can ask about what prof is good for what course, they know good places to study, what to do for fun, the myriad of things that take a semester or two to pick up. But as a xfer by the time you pick this up you're getting close to leaving.
Which brings up my last point, that college is largely over spring of senior year. Sure you still have classes to complete but people are focused on finding jobs and are sometimes burned out on the whole college thing after 3 1/2 years. So as a junior xfer your college experience in some sense lasts 1.5 years at the new school, and a good part of the beginning is spent just learning the ropes and getting integrated into it.