I didn't take the SAT I took the ACT but I'm sure the differences are the same. For the ACT I walked in took the thing once and scored in the 90th percentile. For the MCAT I studied 2 months and still ended up around 3 points below my target score.
maybe I'm a little dumber than the average person on sdn because i had to study REALLY hard for the MCAT and the SAT (but I did much better on the SAT). The MCAT is longer, more complex. It's a different beast all together.
Probably the only things that are even remotely similar are the reading comprehension portions. The MCAT requires you to know far more facts than the SAT. The breadth of knowledge the MCAT tests is far, far beyond what is on the ACT/SAT. It's also about critical thinking and knowledge application rather than just testing to see if you know how to perform a mathematical function presented to you.
They are both testing two different things. The SAT is testing that you have the basics of high school down. Can you do high school math? Did you learn your vocabulary words. Do you have basic reading comprehension. The MCAT is testing that you understand all the medschool prereqs but also that you have critical thinking skills and are able to integrate what you know into novel situations. For instance can you apply your biology concepts to reading a passage about something you DIDN't learn in undergrad and answer a bunch of questions about that passage.
The MCAT you need to walk in with much more factul knowledge. Only on the verbal reasoning section is all the information you need to do well found on the test.
Format is somewhat similar to the reading comprehension and Biology ACT sections. You are given a passage, you read it and answer questions about it. the differences in this are 1. The above where not all info needed is in the passages and 2. There are a few stand alone question that have nothing to do with the passage.
The MCAT is now on a computer, so you should practice accordingly (computer practice tests, reading on a computer ect).
I actually felt that the time limit was much more forgiving on the MCAT, but this could be because I actually prepared for the MCAT.
As far as studying goes I think lots of practices tests are the way to go for both.