I think it's fair to say that MBA programs are more comfortable with GMAT scores, since they have decades of experience with the GMAT, and adcoms have plenty of data on the correlation between GMAT scores and performance in business school. But I don't think that the GRE is going to cause you any particular trouble--if they love your application and your GRE score is solid, you'll be OK. If you were only applying to MBA programs, I would recommend sticking with the GMAT, since it's more familiar to MBA adcoms. But under the circumstances, it doesn't sound like you really have the time and energy to take both tests, and there probably isn't too much downside to sticking with the GRE in your case.
You might want to check out the GRE/GMAT score converter, just to get a sense of where you might stand in relation to the schools' GMAT averages: https://www.ets.org/gre/institutions/ab ... rison_tool
Good luck with everything!
Helping students kick the GMAT in the nuts since 2002... www.gmatninja.com.