This is a really good question! There are many things you should consider when deciding whether to take the GRE or the GMAT. First, you should research the programs you're interested in to see if they prefer that applicants take the GRE or the GMAT. Sometimes the answer will be clear at that point and there will be no further consideration necessary!
If they don't have any preferences, here's what I'd recommend:
1. Find out which test you have a better baseline aptitude for.
If you are curious as to whether you are more suited for the GMAT exam, the most accurate practice test is the one offered by GMAC called GMATPrep. It's the equivalent of PowerPrep II for the GRE. Take both of those exams, and see which one you find easier/more suited to your skill set. You can also make a dispassionate determination by comparing your percentile outcome for each test. If your skills are better showcased on one test over the other, that's the one you should take!
2. Consider the specific content and question styles of each test.
In general, there are a couple of main differences between the GRE and the GMAT. If you want to read a more in-depth writeup, this blog post is useful!
To determine the similarities between the GMAT and GRE materials, you can take a look at the lists of lessons we have available for each exam: GRE vs GMAT. You'll notice that a large part of the material overlaps, with some differences:
- The GMAT has a specific lesson section geared towards data sufficiency (a core part of GMAT quant) because of the tricky nature of this type of question
- For the topics that overlap, the GMAT lessons go into further detail, and have lessons that explain more advanced strategies than the GRE product, though they cover similar math material overall.
- The GMAT focuses more on grammar and correct style; in contrast the GRE emphasizes vocabulary and complicated sentence structure.
The main question types on the GRE are:
- Quant: quantitative comparison, data interpretation, numeric entry, multiple choice, multiple answer
- Verbal: text completion, sentence equivalence, reading comprehension
- Writing: 1 issue essay and 1 argument essay
The main question types on the GMAT are:
- Quant: data sufficiency, problem solving
- Verbal: critical reasoning, reading comprehension, sentence correction
- Integrated Reasoning
- Writing: 1 analysis of argument essay
The biggest difference reported is that GMAT math is generally a bit more difficult than GRE math. Otherwise, your choice of test, if you have an open choice, should come down to your personal strengths.
3. Consider what the test needs to say about you as a candidate.
What skills are the admissions committee members going to be looking for? The GRE is a much more broad, basic graduate aptitude assessment while the GMAT is geared toward business and related fields. If there are certain aspects of your application that are lacking and can be bolstered with correctly selecting the test you take, this is a good way to augment your profile!
Ultimately, if you are in a position to choose, you're likely to find success with either test. Good luck! :)