How big is the difference really? Another month or two travel time? Humans have spent over a year (Mir) and routinely do six month stints on the ISS. If you've got the tech to get to Mars and land there, then an extra month or two flight time is not going to kill you.
I say go as soon as we have the flight hardware built and qualified.
I say go as soon as we have the flight hardware built and qualified.