I definitely agree that it's worth a shot at a practice GRE exam, but it's hard to imagine a scenario in which your GRE score would be wildly better than your GMAT score. Yes, the quant feels much easier on the GRE, but everybody in the admissions office knows that, and will take your GRE score with a grain of salt. ETS (the guys who write the GRE) have a GMAT-GRE score converter on their website, and you might want to take a look at that after you finish your GRE practice test: https://www.ets.org/s/gre/flash/bschool ... index.html
The good news is that the quant section of the GRE contains far less variety than the GMAT (you'll see lots of data analysis, algebra, geometry, and statistics, but only a small sprinkling of other topics), and you can go back and revise your answers within each section. Trouble is, you would need to be extremely accurate to compensate for the fact that the quant questions are, on average, easier than the GMAT. The other problem is that the vocabulary on the GRE can be really, really rough, depending on your skills and background.
I would argue that the GRE would be a better test for you under only two circumstances:
1. You’re better at vocabulary than grammar. I don’t know that I’ve ever met anybody for whom this is the case. Maybe a really well-read native English speaker who lacks the discipline to do well on sentence correction?
2. You’re comfortable with statistics and algebra and geometry, but you struggle on the toughest GMAT-style questions. It’s possible that a test-taker with moderately strong quant skills—and relatively little propensity to make dumb errors—would have a somewhat easier time on the GRE.
Again, I also recommend taking a practice test to see how you do, but I'm convinced that only a very tiny percentage of test-takers will actually gain an advantage by taking the GRE instead. Maybe you're in that tiny percentage?
Good luck with everything!
Helping students kick the GRE and GMAT in the nuts since 2002. http://www.gmatninja.com.