One of my W2's is wrong. Is it better to file using the incorrect W2 info and then submit corrections later or file for an extension and wait for the W2C?

I would suggest you file with the incorrect W-2 and amend later.  If you end up owing IRS, by filing an extension only allow you to file before October 15, 2018.  It does not give you additional time to pay any taxes you may owe, which are still due by April 17 of 2018 (June 15 if you're out of the country).  

If you know you are getting a corrected W2, file for an extension, list the estimated the tax due based on the correction amount if you know it, 

When filing an extension, you are supposed to submit payment, whether or not you may be out of the country.

preparing an amended tax return is a big pain.