I knew early on that I wanted to do a JD/MBA program. That meant that I had to take BOTH the LSAT and the GMAT and I had to do a lot of research before even taking the tests, just to figure out which one I wanted to take first. At first I thought that I would take the GMAT first and warm up to the LSAT, because of the two sections that overlap on the tests (Reading Comprehension and Critical Reasoning) the GMAT is supposed to be easier. I ultimately decided this was not a good idea, since I was aiming to get a top score on both tests. Since the GMAT is computer adaptive, you need to be able to correctly answer the most difficult questions, if you want a high score, so taking the LSAT, which is much more difficult, prepares you for that exactly. Furthermore, there is a danger to getting used to taking easier questions and then having to take more difficult questions. It might hurt you when you are trying to take the LSAT.
After completing both tests with a 174 on my LSAT and a 770 on the GMAT, I think that taking the LSAT first was definitely the best way to go. The LSAT prepares you to ace the GMAT Reading Comprehension and Critical Reasoning questions without even having to study. Wheres the GMAT would have just partially prepared you for the LSAT questions of the same type and you would have had to study a lot more in order to get a good score.
So what are the similarities and differences between the two tests? First of all, the GMAT is computer adaptive, whereas the LSAT is a paper test. They both have an essay, but the essay for the LSAT is taken at the end of the exam and it is not scored. The LSAT also has an entire experimental section, but you don’t know which is which when taking the exam. In total the LSAt has six 35 minute sections: two critical reasoning, one reading comprehension, one logical reasoning (logic games), one experimental section which is either critical reasoning, reading comprehension or logical reasoning, and one essay section. Of those, only four sections are graded and you are scored on a scale of 120-180, which is based on how well you did in comparison to everyone else who took the test at the same time as you. Unlike the GMAT, you can only take the LSAT four times during the year.