What to do after missing the registration deadline for the GRE subject test in mathematics
Although not a direct answer to the question, there is a third option:

Take the October test on standby

From the ETS website:


  Standby testing may be available if sufficient space, test materials
  and staff are available at the test center. If you'd like to attempt
  to test as a standby, report to the test center by 8:15 a.m. with:
  
  a completed Registration Form for the GRE Subject Tests (PDF) payment
  (including the registration and standby testing fee) acceptable photo
  ID Standby registration is not available for Monday testing, testing
  in Mainland China, Hong Kong, Taiwan, Korea, India, or Iran, or those
  requesting disability or health-related testing accommodations.


I don't know how many standby seats are generally available, but it seems worth a shot rather than delaying your studies by a year.