I realize this is probably SDN suicide to post on this thread, but I just wanted to present my own experience.
I took several courses at the University of Iowa during high school in addition to a full load in high school, 2/semester during senior year and 8 total, over the course of three years. These courses included both advanced math courses and general science courses (GenBio/GenPhys) as well as some other courses in neuroscience and philosophy. I breezed through these with minimal effort, studying the night before for exams and completing problem sets the day they were due between classes at school.
For college, I attended an ivy with notorious (justifiably, admittedly) grade inflation. I had to work my butt off to achieve the same kind of grades and %ile-wise, I was still nowhere near where I was in my classes at the University of Iowa.
Now, while the University of Iowa is a fine state school it's doesn't have the same reputation for rigor as say, UC Berkeley. I was also only taking 2 courses (albeit in addition to a full load in high school, which included 4 AP courses each year) before compared to the 4+ in college. Further, it could be said I had more responsibilities in college, with more "demanding" extracurricular a that required more time and dedication. And of course, this is just one person's experience at two schools. Still, I'd be lying if I thought equivalent courses between the two universities required the same amount of work.
HOWEVER, as many people have mentioned, because of large class sizes and most of the times, more stringent grade allocations, better grades are harder to come by at state universities. I had plenty of friends who did just as well as I did during high school, went to state schools for college and applied to medical school. They did fine. Given the MCAT and the countless variables that account for class performance, when it comes to medical school admissions, I've come to believe that for any single individual, regardless of which school they choose to attend, they would have ended up doing just as well in either. If you're a strong candidate, it doesn't matter what school you chose to attend when you were just barely 18. You'll do great either way.
As a side note, to prevent giving anyone about to bite my head off any more ammunition, my college education cost was significantly less than what I would have paid for my in-state tuition. While I would have gotten a solid merit scholarship had I chosen to attend my stare school, my family and I decided that my education was worth the "extra" money (that was still under the normal state tuition levels). I am not a trust fund baby nor do I shop at Brooks Brothers. I am not saying this decision is the right one for very person, but it has worked out for me and I have no regrets -- and as I've said, I think I would have done just fine as well had I gone to the University of Iowa.